One of us prays and does not know how much he has prayed.He replied, "The Messenger of Allah ﷺ said, 'When one of you prays and does not know how much he has prayed, let him perform two prostrations while sitting.'"
حدثنا عمرو بن رافع حدثنا إسمعيل ابن علية عن هشام حدثني يحيى حدثني عياض أنه سأل
أحدنا يصلي فلا يدري كم صلى فقال قال رسول الله ﷺ إذا صلى أحدكم فلم يدر كم صلى فليسجد سجدتين وهو جالس
[Its chain is hasan] Abu Dawud narrated it in the Book of Prayer, chapter: One who says he should complete the prayer based on what seems most likely to him, no. 1029, from the hadith of Ismail with the same wording, and Tirmidhi graded it hasan, no. 396, and Hakim and Dhahabi graded it sahih.