Source: Mujtaba Issue 34 July/Aug 2004
"And whoever argues with you concerning Him, after the knowledge you have been given, then say: 'Come, let us calf our sons and your sons, our women and your women, ourselves and yourselves, then let us sincerely pray and so call the curse of God upon the liars.'" (Surah Aare Imran, Verse 61)
This verse was revealed in 9 A.H. when a group of Christians from Najran came to Madinah to meet the Prophet (s.a.w.w.)
The Prophet (s.a.w.w.) told them about Islam and explained to them that Isa (a.s.) was a Messenger and not God's son. Although the Christians had no answers to his logical explanations, they still refused to accept Islam.
So this verse was revealed and the Christians agreed to a Mubahila (Calling down the curse of God).
On the 24th of Dhul Hijja, the Prophet (s.a.w.w.) arrived carrying Imam Husayn and holding Imam Hassan's hand. Lady Fatimah and Imam Ali (peace be upon them all) followed him. He told them, "When I pray, say Ameen."
When the Christians saw this holy group, their chief said, "0 my people, do not contest with this family, for I see such pure faces that if they ask Allah to move a mountain from its place, He would do it!"
So the Christians changed their minds, and agreed that Najran would pay the jizya tax to the Muslims instead.