The following du`a is recommended after the daily obligatory prayers in the month of Rajab.
The du`a, according to Shaykh `Abbas Qummi in MafateeHul Jinaan has been taught by Imam Ja`far as-Sadiq (a).
 Sayyid Ibn Tawus has narrated that Muhammad ibn Dhakwan—known as ‘al-Sajjad’ due to his remarkable, long prostration during which he used to weep so heavily that he lost his sight—
asked Imam Ja`far al-Sadiq (a.s), saying, “May Allah accept me as ransom for you! We are now in Rajab; I therefore please you to teach me a supplication due to which Almighty Allah may help me.” Hence, the Imam (a.s) asked him to write down the following:
يَا مَنْ أَرْجُوهُ لِكُلِّ خَيْرٍ،
O He Whom alone I hope for all good things;
وآمَنُ سَخَطَهُ عِنْدَ كُلِّ شَرٍّ،
and Whose wrath I do not expect when I do wrong;
يَا مَنْ يُعْطِي الْكَثِيرَ بِالْقَلِيلِ،
O He Who recompenses the little deed with very much reward;
يَا مَنْ يُعْطَي مَنْ سَأَلَهُ
O He Who answers him who begs Him;
يَا مَنْ يُعْطي مَنْ لَمْ يَسْأَلْهُ
O He Who gives those who do not ask Him
وَمَنْ لَمْ يَعْرِفْهُ تَحَنُّناً مِنْهُ وَرَحْمَةً،
and do not even recognize Him out of His kindness and mercifulness:
أَعْطِنِي بِمَسْأَلتِي مِنْ جَمِيعِ خَيْرِ الدُّنْيَا،
(please do) give me, for I pray You, the whole welfare of this world
وَجَمِيعِ خَيْرِ الآخِرَةِ،
and the whole welfare of the Next World;
وَاصْرِفْ عَنّي بِمَسْألَتي إيَّاكَ
and save me, for I pray You,
جَميعَ شَرِّ الدُّنْيا وَشَرِّ الآخِرَة
the whole evil of this world and the Next World
فَإنَّهُ غَيْرُ مَنْقُوصٍ مَا أَعْطَيْتَ،
Verily, all that which You give is unblemished.
وَزِدْنِي مِنْ سَعَةِ فَضْلِكَ يَا كَرِيمُ.
(Please do) increase Your favors upon me, O the All-generous.
يَا ذَاَ الْجَلالِ وَالإكْرَامِ،
O the Lord of Majesty and Honor;
يَا ذَاَ النَّعْمَاءِ وَالْجُودِ،
O the Lord of bliss and magnanimity;
يَا ذَاَ الْمَنِّ وَالطَّوْلِ
O the Lord of favor and munificence;
حَرِّمْ شَيْبَتِي عَلَى النَّارِ.
please do) rescue my white-hair beard from Hellfire.