A man came to the Prophet ﷺ and said: O Messenger of Allah ﷺ, I prayed but I do not know whether I did an even number [of rakʿahs] or an odd number. The Messenger of Allah ﷺ said: ʿBeware lest the Shaitan toy with you in your prayer. Whoever among you prays and does not know whether he did an even number [of rakʿahs] or an odd number, let him prostrate twice, for that will complete his prayer.ʿ It was narrated that Masarrah bin Maʿbad said: Yazeed bin Abi Kabshah led us in praying ʿAsr, then he turned to us after the prayer and said: I prayed with Marwan bin al-Hakam and he did two prostrations like these, then he turned to us and told us that he prayed with ‘Uthman ؓ , who narrated from the Prophet ﷺ ... and he mentioned a similar report.  

جَاءَ رَجُلٌ إِلَى النَّبِيِّ ﷺ فَقَالَ يَارَسُولَ اللهِ إِنِّي صَلَّيْتُ فَلَمْ أَدْرِ أَشَفَعْتُ أَمْ أَوْتَرْتُ فَقَالَ رَسُولُ اللهِ ﷺ إِيَّايَ وَأَنْ يَتَلَعَّبَ بِكُمُ الشَّيْطَانُ فِي صَلاتِكُمْ مَنْ صَلَّى مِنْكُمْ فَلَمْ يَدْرِ أَشَفَعَ أَوْ أَوْتَرَ فَلْيَسْجُدْ سَجْدَتَيْنِ فَإِنَّهُمَا تَمَامُ صَلاتِهِ  

"إيَّاى وَأَنْ يَتَلَغَبَ بِكُمْ الشَّيْطَانُ في صَلَاتِكُمْ مَنْ صَلَّى مِنْكُمْ فَلَمْ يَدْرِ أَشَفع أَمْ وَتْرٌ فَليَسْجُدَ سَجْدَتَيْنِ فَإِنَّهُمَا تَمَامُ صَلاِتهِ".