What is the ruling on the prayer of one who prayed behind an Imam who gave a single Tasleem after the first Tashahhud, then performed a third Rak'ah, then gave a single Tasleem after the second Tashahhud and prostrated for forgetfulness, while the congregant gave the second Tasleem before the Imam's prostration for forgetfulness and did not follow him in it?

The prayer of one who restricts himself to a single salaam is valid according to the majority of scholars, but it is more correct to perform the second salaam because it is considered a pillar by some scholars. Following the Imam in the prostration of forgetfulness (sujud al-sahw) is obligatory, but not following does not invalidate the prayer. The Imam prostrating for forgetfulness twice is contrary to what is correct, but it does not invalidate the prayer if he was ignorant or forgetful. In conclusion, the prayer of the Imam and the congregants is valid.