Mubahila: Prophet (as) brought along dearest ones, proving Islam's veracity
The day of Mubahila [the day Prophet Muhammad (pbuh) challenged Christian missionaries concerning the unity of God] is the day the Holy Prophet of Islam (pbuh) brought the most cherished people in his life along with him [to the event for defending Islam].
An important point concerning the verse "Mubahila"* is that it involves “ourselves and yourselves”; it involves “our women and your women”. Beloved individuals were chosen by the Holy Prophet (pbuh) and stood as proof in a debate, of which, distinguishing the truth from falsehood became the guiding principle to be manifested through everyone.
On no other occasion did the Prophet (s.w.a.) involve his dear ones, his children, his daughter and Imam Ali (a.s.)--his brother and successor--in the arena of promoting Islam and expressing truth. In that way, the day of Mubahila becomes exceptional.
It [Muhabila] reveals that expressing the truth, promoting the truth is extremely important. He [Mohammad (pbuh] presents them [his loved ones] before his challengers, in order to invoke God's curse on deceivers. He challenges the opponents; saying any of us who is righteous may remain and the side against the truth may perish by God’s damnation
The same thing happened during the month of Muharram, when Imam Hussein (a.s.) brought his dear ones to the desert of Karbala in order to clarify the truth. He knew what was going to happen, yet he brought Zeinab, his wives, his children, and his brothers with him. Like the day of Mubahila, it was again a matter of clarifying the truth and promoting Islam, in the real sense of the word. His actions reveal the importance of promoting Islam. "If someone comes across a ruthless sultan who mixes up halal with haraam and breaks his promise to God, they deserve living with the sultan in the hereafter." That is to say, when someone sees such a person, they should clarify the truth through their words and actions, just as Imam Hussein (a.s.) did; and, he paid the price for it. Imam Hussein (a.s) brought his dear ones, his wives, and children to Karbala in order to clarify the truth.
Ayatollah Khamenei, Dec 13, 2009
* "Come! Let us gather together - our sons and your sons, our women and your women, ourselves and yourselves. Then let us earnestly pray, and invoke the curse of Allah on those who lie!" [The Holy Quran, 3: 61]