The Prophet Muhammad (S) has said, “O' ‘Ali! The person whom, others are scared of his tongue (what he says), shall be of the people of the hell fire. O' ‘Ali! The worst of people is the one whom others show respect to out of fear of his wickedness.
O' ‘Ali! The worst of people is the one who sells his next life for his life of the transient world and one who is even worse than this is the one who sells his next life for the life of the transient world of other people!”[1]
Without doubt, the points mentioned in this tradition are very moving and if we did not have even a single proof for the truthfulness of the status (of Prophethood) of the Prophet Muhammad (S), then this saying would be sufficient for an individual to realize the great status and rank of this individual!
As the historians narrate, the Prophet Muhammad (S) grew up within a society steeped in ignorance, never went to school and never wrote anything, however such words which grant wisdom to others have been heard flowing from his blessed lips! All of this points to his greatness and station of Prophethood and through such sayings, a person is guided to understand that everything he has spoken are indeed miracles.
In the saying quoted above, we see that he has explained the transient world in great detail.
In the first sentence, he has pointed us to issues in regards to a person's tongue and what he says with it.
The scholars of etiquette tell us that whatever is said about the tongue and the sins and evils which stem from is never enough. A great number of difficulties, tribulations, bad states - and even good states we find ourselves in, our victories and defeats all come through reforming our tongue and what we speak - and how difficult it is to reform the tongue!
The Noble Prophet (S) has stated, “O' ‘Ali! The person whom others are scared of his tongue and what he says shall definitely be of the people of the hell fire.”
The following example best illustrates this line of the tradition. Sometimes, we see a person enter into a gathering and immediately, everyone present falls into a state of fear lest the person should begin to speak, since they have all felt the bite of his tongue and his harsh words.
The tongue is something very dangerous - of course however, we must realize that the importance of the pen and what it writes is no less than what the tongue speaks, and it actually may even be more dangerous![2]
In the second sentence, the Prophet (S) stated, “O' ‘Ali! The worst of people is the one whom people show respect to out of fear of his evilness.”
As we know, people generally show respect to people for two reasons:
1) The individual's moral character and worthy traits and since they benefit from being in the company of such people.
2) They want to repel the dangers and threats which these people can cause. Truthfully, others are scared that if they do not show respect to a particular person, then that person shall do something bad to them!
There is a story which has been related about a person who had stated, “I will give the wealth in the public treasury to two groups of people: one group are those who who follow and respect me only because they benefit materialistically from me, while the other group are those who respect me since they want to repel the evil which I can cause them!”
Question: Why is such a person as mentioned in the tradition the worst person?
Answer: His sins are actually much greater in number compared to others since he is harming others by compelling them to act contrary to their own desires by forcing them to show respect.
If such a person was to gain wealth from his actions, then without doubt, this wealth would be forbidden and he would not be permitted to use of it! If such a person was to give an amount of money from the public treasury to a person so that the he would not embarrass him in public (out of fear) then that money would be similar to eating the flesh of the swine!
In the third sentence in the tradition, the Prophet (S) has stated, “O' ‘Ali! The worst of people is the one who sells his next life for his life of the transient world…”
In this part of the saying, the Prophet (S) means to say that such a person sells the true life which is in the next world for the transient and short-lived life on Earth, even though:

وَإِنَّ الدَّارَ الأَخِرَةَ لَهِيَ الْحَيَوَانِ

“And the life of the next world that is the (true) life…”[3]
In reality, the transient world which we are in is a form of slow death and how bad it is that one would trade the ever-lasting life of the next world for the temporary, fleeting, base pleasures of the life of this transient world - which is, as mentioned, a life of slow death!
A person who does this is one who, from the point of view of his intellect, is a bad person since he himself is thinking bad thoughts (of the life of this world). From the point of view of his actions, he is also a bad person since he does bad deeds for the life of this world. In addition, from the point of view of his ambitions and aspirations, he is also bad since he has degraded his ambitions and goals!
Even worse than the one mentioned is the person who sells the life of the next world for the life of the transient world - however for other people! These sorts of people are not few in number and are people who say, “I am ready to sell my life of this world and the next world and everything which I possess (both materialistically and spiritually) in order to ensure that another person is comfortable in this life.” Truly, this is the pinnacle of stupidity and is the greatest loss for a person!
Sometimes a person will fall into the trap (mentioned in this tradition) and will open his eyes and see that he is now the physical manifestation of the first part of this tradition (that he has sold the life of the next life for the pleasures of this world).
Slowly, he would realize that he has also become a manifestation of the second part of the tradition (that he has also sold the life of the next world for others around him and for them to be comfortable in this world)! Do not think that a person is forewarned of this or that some signs are given to a person to make him aware of this - no this is not the case! All of a sudden he would notice what has occurred and at that time, he would realize that he is drowning and his hands and feet are tied and that he has no way to get out!

The Dangers of the Tongue in the Islamic Narrations
1. The Noble Prophet of Islam (S) has said:

نَجَاتُ الْمؤْمِنِ حَفْظُ لِسَانِهِ

“The salvation of the true believer lies in protecting his tongue.”[4]
2. Imam Ja’far b. Muhammad as-Sadiq (as) has said, “Luqman said to his son:

يَا بُنَيَّ إِنْ كُنْتَ زَعَمْتَ أَنَّ الْكَلاَمَ مِنْ فِضَّةِ، فَإِنَّ السُّكُوتَ مِنْ ذَهَبٍ

“O' my son! If you think that speaking is silver, then you must know that silence is golden.”[5]
3. The Noble Prophet of Islam (S) has said:

بَلاَءُ الإِِنْسَانِ مِنَ اللِّسَانِ، وَ سَلاَمَةُ الإِنْسَانِ فِي حِفْظِ اللِّسَانِ

“The tribulation of a person lies in his tongue and the protection of a person is in safe-guarding his tongue.”[6]
4. The final Prophet of Allah (S) said to Ibne Mas’ud:

عَلَيْكَ بِحِفْظِ لِسَانِك، فَإِنَّ اللٌّهَ تَعَالـى يَقُولُ أَلْيَوْمَ نَخْـتِمُ عَلى أَفْوَاهِهِمْ وَتُكَلِّمُنَا أَيْدِيهِمْ وَتَشْهَدُ أَرْجُلُهُمْ بِمَا كَانُوا يَكْسِبُونَ

“I advise you to protect your tongue since surely Allah, the Most High has said, (On that day, We shall seal their mouths and their hands will speak to us and their feet shall bear witness as to what they had earned.)”[7]
5. The Commander of the Faithful, ‘Ali b. Abi Talib (as) has said:

طُوبـى لِمَنْ أَنْـفَقَ الْفَضْلَ مِنْ مَالِهِ وَ أَمْسَكَ الْفَضْلَ مِنْ كَلاَمِهِ

“Glad tidings to that person who gives out goodness from his excess wealth and prevents himself from speaking excess words.”[8]
6. It has been narrated from Imam Ja’far b. Muhammad as-Sadiq (as) from his fore-fathers (as) that the Messenger of Allah (S) said:

زِيـنَةُ الْحَدِيثِ الصِّدْقُ

“The beauty of speaking is truthfulness (in speech).”[9]
7. The Messenger of Allah (S) has said:

فِـتْـنَةُ اللِّسَانِ أَشَدُّ مِنْ ضَرْبِ السَّيْفِ

“The tribulation of the tongue (what is spoken) is greater than the striking of the sword.”[10]
8. It has been narrated from ‘Abdullah b. Sanan from Imam Ja’far b. Muhammad as-Sadiq (as) that the Messenger of Allah (S) said:

أَلاَ أُنَـبِّئُكُمْ بِشِـرَارِكُمْ؟ قَالَ أَلْمَشَّاؤُونَ بِالنَّمِيمَةِ، أَلْمُفَرِّقُونَ بَيْنَ الأََحِبَّةِ، أَلْبَاغُونَ لَلْبِرَآءِ الْمَعَايِبِ

“Shall I not inform you of the worst amongst you?” The people replied, “O' Prophet of Allah! Go ahead!” The Prophet said, “It is those who: go forth as tale-bearers (those who indulge in gossip); cause separation amongst people who love one another; and make excuses for sins (which they commit).”[11]
9. Imam Ja’far b. Muhammad as-Sadiq (as) has said:

إِيَّاكُمْ وَالْمِزَاحَ فَإِنَّهُ يَذْهَبُ بِمَآءِ الْوَجْهِ وَمَهَابَةِ الرِّجَالِ

“I advise you to stay away from joking with others since this act removes a persons's worth and honour.”[12]
10. Imam Ja’far b. Muhammad as-Sadiq (as) has said:

إِنَّ أَبْغَضَ خَلْقِ اللٌّهِ عَبْدٌ اتَّقِيِ النَّاسُ لِسَانَهُ

“Surely the worst of creations of Allah is the servant whom other people are afraid of his tongue (what he says).”[13]!

Notes:
[106] Bihar al-Anwar, vol. 74, pg. 46
[107] A short discussion about the pen and its importance - The role of the pen in the life of human beings:
The movement of the pen on paper records the outcomes of humanity. The victories and defeats of the society of humanity is contained and preserved in the tips of the pens; the pen is the protector and safe-guarder of the sciences and knowledge; it is the assistant to the thoughts of the scholars and researchers; it is the link of the thoughts of the scholars and the bridge for humanity which links the past to the present; even the heavens and the earth are linked through the Divine Tablet and the Pen!
The pen is the instrument which unites people who have been separated by time and space and just as it is said that all of the great scholars and thinkers of the world of humanity throughout the entire course of history and in all areas around the world are gathered in one large library (their thoughts and beliefs which have been preserved on paper and then published in books which are then gathered and collected in libraries)!
The pen is the confidant of a person; it is the treasurer of the sciences and the collector of all the experiments and research of the centuries and ages. If we see that the Quran has taken an oath by the pen, then it is for all of these reasons since the Quran only swears and takes an oath by things which are very great and valuable!
The leaders of the faith of Islam, in various traditions (Wasaaf1il ash-Shia, vol. 18, pg. 56, no. 14 to 20) have emphasized to their companions and close associates to not rely upon their memory. Rather, they were always instructed to write down and record the Islamic narrations and Divine sciences and teachings and to preserve these for the future generations.
In Tafsir Majma al-Bayan (vol. 1, pg. 332) it has been mentioned that some of the scholars have stated:

أَلْبَيَانُ بَيَانَانِ: بَيَانُ اللِّسَانِ، وَ بَيَانُ الْبَنَانِ و بَيَانُ اللِّسَانِ تَدْرُسُهُ الأََعْوَامِ، وَ بَيَانُ الأَقْلاَمِ بَاقَ عَلى مُّرِ الأََيَامِ
 

“Communcation is of two types: communication with the tongue and communication with the pen. The communication carried out with the tongue becomes old with the passing of time and are eventually removed from memory, however the communication carried out with the pen (that which is written and recorded) remains for eternity.” Tafsir-e-Namuna (summarized), vol. 24, pg. 373-376
[108] Surat al-Ankabut (29), verse 64
[109] Wasail ash-Shia, vol. 8, pg. 533
[110] Bihar al-Anwar, vol. 71, pg. 297
[111] Ibid, vol. 71, pg. 286
[112] Ibid., vol. 77, pg. 109
[113] Ibid., vol. 71, pg. 238
[114] Ibid., vol. 71, pg. 17
[115] Ibid., vol. 71, pg. 286
[116] al-Kafi, vol. 2, pg. 369
[117] Ibid., vol. 2, pg. 665
[118] Ibid., vol. 2, pg. 323