Among the delegations that came to Prophet Muhammad was on e from the Christians of Najran after their chief chief bishop and his people were invited to Islam. They came to negotiate a truce with the Prophet.
They offered terms that were unacceptable to the Prophet, and he invited them to a Mubahila, a prayer for divine curse upon the liars. The Prophet accepted, and a verse from the Quran was revealed telling each side to call their sons, women, and selves to invoke the curse.
While the Christians brough all their leaders, Prophet Muhammad brought only his daughter Fatima, his cousin Ali, and his grandsons Hasan and Husayn. The Christians were struck with fear at seeing their luminous faces and did not go through with the Mubahila.
The lecture goes on to discuss how this event highlights the importance of the family of the Prophet, comparing both Sunni and Shia views.
Lecture notes available at http://www.why-quran.org/?p=742.
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