Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Abdur Rahman bin Awf (may Allah be pleased with him) that he said: The Noble Prophet (peace and blessings of Allah be upon him) said: "When a person forgets in prayer and does not know whether he has prayed one rak‘ah or two, he should base his prayer on one. And when he does not know whether he has prayed two or three, he should base it on two. Similarly, when he is in doubt between three and four, he should consider three as certain, and (after completing the prayer accordingly) perform two prostrations before the salutation."

Sayyiduna Abdullah bin Buhaina (may Allah be pleased with him) says: The Messenger of Allah (peace and blessings be upon him) led us in a prayer (and according to one narration, the Zuhr prayer), and after two rak‘ahs, instead of sitting for tashahhud, he stood up. (To alert him) "Subhan Allah" was said, but when the Prophet (peace and blessings be upon him) had stood up straight, he continued the prayer and did not return. The people also stood up following him. The Prophet (peace and blessings be upon him) continued the prayer until he was about to finish it, with only the salam remaining, and the people were also waiting to say the salam. The Prophet (peace and blessings be upon him), due to forgetfulness before the salam, performed two prostrations while sitting and said « اللہ اكبر» for each prostration, then said the salam. The people also performed the prostrations with the Prophet (peace and blessings be upon him).

It is narrated from Sayyiduna Mughirah bin Shu’bah (may Allah be pleased with him), he says that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When the imam (forgets and instead of sitting for tashahhud) begins to stand up after two rak‘ahs, if he remembers before standing up straight, he should sit down; and if he stands up straight, then (he should continue with the third rak‘ah and) not sit for tashahhud, and (because the middle tashahhud was missed, before the salam) he should perform two prostrations."

Iyad bin Hilal says: I asked Sayyiduna Abu Sa'id: A man performs prayer but (due to forgetfulness) he does not know how many units he has prayed? He said: The Messenger of Allah (peace and blessings be upon him) said: "When any one of you performs prayer and does not know how many units he has prayed, then while sitting, he should perform two prostrations."

Sayyidah Aisha (may Allah be pleased with her) narrates that the Messenger of Allah (peace and blessings be upon him) said: "The two prostrations performed due to forgetfulness suffice for every kind of deficiency or excess that occurs in the prayer."