It is narrated from Hazrat Ata bin Yasar that the Messenger of Allah, peace and blessings be upon him, said: "When any of you is in doubt during prayer and does not remember whether he has prayed three rak'ahs or four, then he should pray one more rak'ah and perform two prostrations before the salam. Then, if this rak'ah that he has prayed is actually the fifth, it will become a pair with these prostrations, and if it is the fourth, these prostrations will be a humiliation for Satan."

It is narrated from Hazrat Salim bin Abdullah that Sayyiduna Abdullah bin Umar, may Allah be pleased with them both, used to say that when any one of you has doubt in his prayer, he should consider what he has forgotten, then recite it, and then perform two prostrations of forgetfulness while sitting.

It is narrated from Hazrat Ata bin Yasar that I asked Sayyiduna Abdullah bin Amr bin al-As and Sayyiduna Ka'b bin Ahbar (may Allah be pleased with them both) about a person who doubts in his prayer and does not remember whether he has prayed three rak'ahs or four. Both replied that he should pray one more rak'ah and then, while sitting, perform two prostrations of forgetfulness (sujood as-sahw).

It is narrated from Hazrat Nafi that when Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) was asked about forgetting in prayer, he said: He should think about what he has forgotten and then recite it.