Ref. No. 1204/42-506
In the name of Allah the most Gracious the most merciful
The answer to your question is as follows:
‘Mubahala’ means cursing each other. Two people or two groups who consider themselves to be right, both plead with each other to the God and want God Almighty to curse the liar so that it becomes clear to everyone which one is on the right path. The incident of Mubahala has been narrated in Surah Al-Imran of the Holy Qur'an that the Prophet of Allah (saws) sent a decree to the Christians of ‘Najran’ in which three things were mentioned in order as follows: (1) embrace Islam (2) pay the jizya (tax) (3) or get ready for war.
The Christians, in consultation with each other, sent Shurahbeel, Abdullah ibn Shurahbeel, and Jabbar ibn Qais to the Prophet of Allah (saws). They came and discussed religious matters until they had a heated argument in proving the divinity of Jesus. At the same time, this verse was revealed about Mubahala. The Prophet of Allah (saws) invited the Christians for Mubahala and they accepted the invitation at that time. But later, they avoided coming to Mubahala because they saw that the prophet of Allah (saws) had brought his closest people, his daughter Fatima Zahra, his son-in-law Imam Ali, his grandchildren Hassan and Hussain etc.
So Shurahbeel said to his two companions, "You know that this is Allah’s apostle. If we argue with the prophet of Allah (saws) then certainly it will destruct and ruin us. So find another way of salvation. Finally, they accepted the offer of peace and thus the prophet's legitimacy was revealed to all. (Maariful Quran: 2/85)
And Allah knows best
Darul Uloom Waqf Deoband