Long ago in Arabia, there was to be a debate between Mohammad (PBUH) and Christians of Najran.

The debate was traditionally known as Mubahala, which occurred on the 10th year of AH. The prophet (PBUH) decided to participate in Mubahala and took up an invitation to meet with the Christians. During the event each side was to condemn the liar, asking God for punishment. However, when the Christian delegation saw the immaculate faces of the Prophet (PBUH) and his family (Ahlul Bait) including Imam Ali, Lady Fatima Zahra, Imam Hassan, and Imam Hussain (PBUT), they were astonished; thus, they refrained from seeking condemnation towards the prophet and his household (PBUT). They knew that the prophet’s dua and his family’s Duas (PBUT) would be accepted, so the Bishop of Najran asked the prophet (PBUH) for forgiveness and compromise. Following are the statements of Ayatollah Khamenei on this event.

Source: en.shafaqna.com