ENOfficial
‘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.’”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَمْرُو بْنُ رَافِعٍ، حَدَّثَنَا إِسْمَاعِيلُ ابْنُ عُلَيَّةَ، عَنْ هِشَامٍ، حَدَّثَنِي يَحْيَى، حَدَّثَنِي عِيَاضٌ، أَنَّهُ سَأَلَ أَبَا سَعِيدٍ الْخُدْرِيَّ فَقَالَ أَحَدُنَا يُصَلِّي فَلاَ يَدْرِي كَمْ صَلَّى . فَقَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " إِذَا صَلَّى أَحَدُكُمْ فَلَمْ يَدْرِ كَمْ صَلَّى، فَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ " .
sunnah.com