'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.'"
English reference: Vol. 1, Book 5, Hadith 1204
Arabic reference: Book 5, Hadith 1261