Mubahila is when two opposing parties who have a difference on any issue send the curse of Allah on each other and ask God to send punishment on those who are liars.
The Christians of Nadran first agreed to a mutual curse and agreed that the next day they would make a mubahil at the appointed place. They left the prophet, and when they all gathered together and began to talk about the future matter, the chief priest said: “If tomorrow Muhammad comes to the appointed place with his family, do not agree to mutual curses. If he comes with companions, then there is nothing to worry about. So he is a real prophet.” The next day, the Messenger of Allah, along with Ali, Fatimah, Hasan and Husayn, came to the appointed place for the mubahil. The Prophet said to his family: “When I turn to Allah with a request, you say ‘Amen.’ The Christians, seeing those accompanying the Prophet, were very frightened and confirmed that he was acting in a similar way to the method of other prophets. They began to ask the Prophet not to carry out mutual curses and make peace with them, and this day is called the day of Mubakhil.