What is the ruling on the prayer of a disbeliever—whether written or unwritten—individually or in congregation with Muslims, in the mosque or elsewhere?

The prayer of a disbeliever is not valid, nor is he rewarded for it, by consensus, due to the lack of the condition of Islam. Scholars have differed on whether a disbeliever becomes a Muslim if he prays. The Hanbalis and Al-Shafi'i, in one opinion, held that he is judged to have embraced Islam. The Shafi'is, in the well-known opinion of their school, held that he does not become a Muslim if he prays. However, if he prays and the two testimonies of faith (Shahadatayn) are heard from him, then he is judged to have embraced Islam.