'Iyadnarrated that he asked Abu Sa'eed Al-Khudri:
"One of us praysandhe does not know how many (Rak'ah) he has prayed." He said: "TheMessenger of Allah (ﷺ) said: 'When anyone of you prays and doesnotknow how many he has prayed, let him perform two prostrationswhile heis sitting.'" (ID)
English reference (ID): Vol. 1, Book 5, Hadith 1204 (ID)
Arabic reference (ID): Book 5, Hadith 1261 (ID)