'Mubahala' was a tradition used in the days of the Prophets. It involved two groups of people praying to God to punish the group that is making lies about God.
The event of Mubahala is a landmark in Islamic history, and occurred when the Christians came to Medina to have a debate with the Prophet Mohammad about Christianity and Islam.
However, during the debate, the Christians started to question the Prophet's claim to Prophethood and they told him if he is being truthful then he should give them a sign so that they can believe.
Allah then sent a verse from the Quran to Prophet Mohammad:
بِسْمِ اللَّـهِ الرَّحْمَـٰنِ الرَّحِيمِ
" And whoever disputes with you concerning him, after what has come to you of knowledge say: 'Come! Let us call our sons and your sons, our women and your women, and our selves and your selves, then let us invoke in earnest and lay the curse of Allah upon the liars."
Through this verse Allah showed Prophet Mohammad how to deal with this group of Christians by having a contest in which both sides pray that God's curse and punishment should fall on the side which is lying so that the liar is exposed.
The Prophet Mohammad told the Christians that is what they can do and the discussion was ended.
This suggestion by the Prophet surprised the missionaries. They went away and asked their Cheif Monk if they should accept the Prophet's request. The Chief Monk told them that if they go on the day of the Mubahala and find Mohammad with the large crowd of people, that means he is scared and he wants others to pray with him. He told them that this would show that he is not a real Prophet and they can perform the Mubahala.
However, he told them that if they find him with a small number of people, then they should give up the Mubahala and make a settlement with him.
The Christians therefore accepted the request to perform the Mubahala and they decided on a time and place to perform it. The day this contest was held is now known as the day of Mubahala
On the day of Mubahala they saw that the Prophet of Islam (S) entered the chosen place followed by two boys, a young man and a lady. Those two boys were Hassan and Husayn, (as) the young man was Ali-ibn- Abitalib (as), and the lady was Fatimah (as), the Prophet's daughter.
When the Chief Monk saw them, he was shocked and said:
"By God! I see the faces that, if they pray to God for mountains to move from their places, the mountains will immediately move."
"If they curse you, you will be wiped out of existence to the last day of the life of the earth."
The leaders of the Christians rushed to the prophet begging for mercy and offered him a treaty of peace, and that he is to ensure them protection and that they gave him tribute.
By that great event Allah and his messenger made us understood the importance of Fatima al Zahra, Imam Ali, Imam Hassan and Imam Hussain. It became clear after this that these members of his household were the assistants and the associates of the Messenger in inviting people to the Truth and that their prayers are at the same level as that of the Holy Prophet Mohammad.