“One of us prays and he does not know how many (Rak’ah) he has prayed.” He said: “The Messenger of Allah (ﷺ) said: ‘When anyone of you prays and does not know how many he has prayed, let him perform two prostrations while he is sitting.’”
Sunan Ibn Majah Book 5, Hadith 402
← Previous Hadith “The Messenger of Allah (ﷺ) prayed, and he added or omitted something.” (One of the narrators) Ibrahim said: “The confusion stems from me (i.e., he was not sure which it was).” “It was said to him: ‘O Messenger of Allah! Has something been added to the prayer?’ He said: ‘I am only human, I forget just as you forget. If anyone forgets, let him perform two prostrations when he is sitting (at the end).’ Then the Prophet (ﷺ) turned and prostrated twice.”

Sunan Ibn Majah Book 5, Hadith 401 Next Hadith → “(Once) the Prophet (ﷺ) prayed Zuhr with five Rak’ah, and it was said to him: ‘Has something been added to the prayer?’ He said: ‘What is that?’ They told him, and he turned back towards the Qiblah and performed two prostrations.”

Sunan Ibn Majah Book 5, Hadith 403