The Permissibility of Prostration for Forgetfulness Before and After the Closing Salam

Q: Some imams perform the prostration for forgetfulness after the closing salam, some perform it before the closing salam, and some perform it once before the closing salam and once after it. When is the prostration prescribed before the closing salam? When is it prescribed after it? And is the prostration prescribed before or after the closing salam on the basis of obligation or recommendation?

A: The matter is broad in this regard, so both actions are permissible: the prostration before the closing salam and after it; because the hadiths came with this from the Prophet (peace and blessings be upon him).
But the better is that the prostration for forgetfulness be before the closing salam, except in two cases:
The first is: if he says the closing salam due to a deficiency of one rak'ah or more, then the better is that the prostration for forgetfulness be after completing the prayer and saying the salam from it, following the Prophet (peace and blessings be upon him) in that; because the Prophet (peace and blessings be upon him), when he said the closing salam due to a deficiency of two rak'ahs in the hadith of Abu Hurayrah (may Allah be pleased with him) and a deficiency of one rak'ah in the hadith of 'Imran ibn Husayn (may Allah be pleased with them both), prostrated for forgetfulness after the completion and the salam.
The second case is: if he doubts in his prayer and does not know how many he prayed—three or four in the four-rak'ah prayer, or two or three in the sunset prayer, or one or two in the dawn prayer?—but one of the two matters prevails in his opinion—whether it is the deficiency or the completion—then he builds upon what prevails in his opinion, and his prostration is after the closing salam on the basis of preference; due to the hadith of Ibn Mas'ud mentioned in the answer to the previous question. And Allah is the possessor of success.