‘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.’”

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 ﷺ ـ " إِذَا صَلَّى أَحَدُكُمْ فَلَمْ يَدْرِ كَمْ صَلَّى فَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ " .