Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Mughirah bin Shu’bah (may Allah be pleased with him) that he said: 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. But if he has already stood up straight, then he should not sit (for tashahhud), rather he should continue with the third rak‘ah, and (because the middle tashahhud was missed, before the salam) he should perform two prostrations (of forgetfulness).”

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).