"The Messenger of Allah ﷺ prayed and did more or less (rak'ahs). When he had said the salam we said: 'O Messenger of Allah ﷺ, has there been some change concerning the prayer?' He said: 'Why are you asking?' So we told him what he had done. He turned back toward the Qiblah and prostrated two prostrations of forgetfulness, then he turned to face us and said: 'If there had been some change concerning the prayer I would have told you.' Then he said: 'Rather I am a human being and I forget as you forget. If any one of you is not sure about his prayer, let him estimate what he thinks is correct, and complete his prayer on that basis, then say the taslim and prostrate two prostrations of forgetfulness.'" (Using translation from Nasāʾī 1243)   

صَلَّى رَسُولُ اللهِ ﷺ صَلَاةً فَزَادَ فِيهَا أَوْ نَقَصَ فَلَمَّا قَضَى الصَّلَاةَ قَالُوا يَا رَسُولَ اللهِ هَلْ حَدَثَ فِي الصَّلَاةِ شَيْءٌ؟ قَالَ وَمَا ذَاكَ؟ قَالَ فَذَكَرْنَا الَّذِي فَعَلَ فَثَنَى رِجْلَهُ ثُمَّ اسْتَقْبَلَ الْقِبْلَةَ فَسَجَدَ سَجْدَتَيْنِ ثُمَّ أَقْبَلَ عَلَيْنَا بِوَجْهِهِ فَقَالَ لَوْ حَدَثَ فِي الصَّلَاةِ شَيْءٌ لَأَنْبَأْتُكُمْ وَإِنَّمَا أَنَا بَشَرٌ أَنْسَى كَمَا تَنْسَوْنَ فَإِذَا نَسِيتُ فَذَكِّرُونِي وَأَيُّكُمْ مَا نَسِيَ فِي صَلَاتِهِ فَلْيَتَحَرَّ الَّذِي يَرَى أَنَّهُ الصَّوَابُ فَلْيُتِمَّ عَلَيْهِ ثُمَّ يَسْجُدْ سَجْدَتَيِ السَّهْوِ  

[Machine] That the Messenger of Allah ﷺ made a mistake in prayer and performed the two prostrations of forgetfulness. Muslim narrated this in Sahih from Abu Kurayb and others from Abu Muawiya.  

أَنَّ رَسُولَ اللهِ ﷺ سَهَا فِي الصَّلَاةِ فَسَجَدَ سَجْدَتَيِ السَّهْوِ وَرَوَاهُ مُسْلِمٌ فِي الصَّحِيحِ عَنْ أَبِي كُرَيْبٍ وَغَيْرِهِ عَنْ أَبِي مُعَاوِيَةَ