Glorious Quran Chapter 3 Verse 61 says: And unto him who disputeth with thee therein after the knowledge hath come unto thee, Say! (O’ Our Apostle Muhammad!) ( Unto them ) come ye, let us summon our sons, and ( ye summon ) your sons, and ( we summon ) our women and ( ye ) your women, and ( we summon ) ourselves and ( ye ) yourselves and then let us invoke the curse of God on the liars!
This verse refers to the famous event of ' Mubahila ' which took place in the year 10 A.H against the Christians of Najran. A deputation of 60 Christians of Najran headed by Abdul Masih their chief monk priest came and discussed with the Holy Prophet, peace be upon him and his pure progeny, the personality of Prophet Jesus, peace be upon him. The Holy Prophet, peace be upon him and his pure progeny, told them not to deify Jesus for he was only a mortal created by God, and not God Himself.
The place where Mubahila took place between the Ahlul Bayt [a] and the Christians of Najran. In 1997, the Saudi govt started construction of Masjid Banu Muawiya at this place.
Then they asked who the father of Jesus was. By this, they thought that since he was born without a father the Holy Prophet, peace be upon him and his pure progeny, would helplessly accept Jesus' father being Allah himself. In reply to this question was revealed the Verse:
“Verily, similitude of Jesus with God is as the similitude of Adam; He created him out of dust then said He unto him BE, and he became.”
When the Christians did not agree to this line of reasoning, then the verse of Mubahila “And unto him who disputeth with thee therein after the knowledge hath come unto thee, Say ! ( O' Our Apostle Muhammad ! ) ( Unto them ) come ye, let us summon our sons, and ( ye summon ) your sons, and ( we summon ) our women and ( ye ) your women, and ( we summon ) ourselves and ( ye ) yourselves and then let us invoke the curse of God on the liars !” was revealed enjoining upon the Prophet, peace be upon him and his pure progeny, to call the Christians to Mubahila.
To this the Christians agreed and they wanted to return to their place and would have the Mubahila the next day.
Early next morning the Holy Prophet, peace be upon him and his pure progeny, sent Salman al-Farsi, May Allah be well pleased with him, to the open place, fixed outside the city for the historic event, to erect a small shelter for himself and those who intended to take along with him for the contest.
On the opposite side appeared the Christian priests, while at the appointed hour the Christians witnessed the Holy Prophet, peace be upon him and his pure progeny, entering the field with Imam Hussain, peace be upon him, in his lap, Imam Hasan, peace be upon him, holding his finger, and walking beside him Sayedah Fatima, peace be upon her, and followed by Imam Ali al Murtaza, peace be upon him. The Prophet, peace be upon him and his pure progeny, on reaching the appointed spot stationed himself with his daughter, her two sons and her husband, raising his hands towards the heaven said:
“Lord these are the People of my House.”
The Chief Monk on knowing that the baby in the lap of the Prophet, peace be upon him and his pure progeny, was his young grandson Imam Hussain, peace be upon him, the child walking holding the hand of Prophet, peace be upon him and his pure progeny, was his first grandson Imam Hasan, peace be upon him, the Lady behind him was daughter Fatima, peace be upon her, the mother of the two children and the one who followed the Lady was the husband of Fatima, peace be upon him, addressed the huge crowd of the people who had gathered on the spot, and addressed them saying:
“By God, I see the faces which, if they pray to God for mountains to move from their places, the mountains will immediately move!
O believers in the Jesus of Nazareth, I will tell you the truth that should ye fail to enter into some agreement with Muhammad and if these souls whom Muhammad has brought with him, curse you, ye will be wiped out of existence to the last day of the life of the earth!”
The people readily agreed to the advice counseled by their Leader. They beseeched the Holy Prophet, peace be upon him and his pure progeny, to give up the idea of the agreed Mubahila and requested for themselves to be allowed to continue their faith, offering to pay ' Jizya '.
- At-Tabari, Commentary of the Quran, v 2 p 192 -> 193
This historic event of a unique triumph of Islam is taken by the followers of Ahlol Bayt, peace be upon them, as a religious thanksgiving festival of the triumph against falsehood. Some of the significance of this event are as follows:
* This event un-questionably establishes the truth about the spiritual purity of the Ahlol Bayt, peace be upon them,
* It proves beyond any doubt as to who are the members of the house of the Prophet, peace be upon him and his pure progeny,
* The seriousness and the solemnity of the occasion demands absolute purity, spiritual as well as physical in the individuals to serve in the fateful occasion for the Holy Prophet, peace be upon him and his pure progeny, to present them to God as the best one of His creation to be heard in the prayers of Truth!

References
Ghayatul Maryam, p 300
Sahih Muslim, v 4 p 1285 (English Edition)