‘Iyad narrated that he asked Abu Sa’eed Al-Khudri:“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.’”

English translation · Sunan Ibn Majah