What is the ruling on someone who forgot the prostration of forgetfulness (sujud al-sahw) and remembered it after finishing the prayer and rolling up the prayer rug; is it permissible for him to return and perform the prostration of forgetfulness?

There is no consideration for doubt and obsessive thoughts regarding the validity of prayer in three cases: if the doubt occurs after finishing the prayer, unless the worshipper is certain of an increase or decrease. And if the obsessive thoughts remain within the realm of delusions and do not rise to the level of doubt. And if the worshipper is frequently afflicted with obsessive thoughts and doubts (a sickness).

As for cases other than these, if the worshipper doubts the number of rak'ahs, he should build upon what is certain and perform two prostrations before the taslim, as the Prophet, peace and blessings be upon him, said: "If one of you doubts in his prayer and does not know whether he prayed three or four rak'ahs, he should cast away the doubt and build upon what is certain, then perform two prostrations before he gives the taslim." If he doubts having performed a prostration or a bowing, he should perform it and then prostrate for forgetfulness (sujud as-sahw). If he forgets to prostrate for forgetfulness before the taslim, he should prostrate for it afterwards, as long as he has not left the mosque or invalidated his ablution. The prostration for forgetfulness is waived if he remembers it after a long interval, such as leaving the mosque.