The Messenger of Allah ﷺ prayed five rak’ahs in the noon prayer. He was asked whether the prayer had been extended. He asked what they meant by that. The people said : you prayed five rak’ahs. Then he made two prostrations after having given the salutation.  

صَلَّى رَسُولُ اللَّهِ ﷺ الظُّهْرَ خَمْسًا فَقِيلَ لَهُ أَزِيدَ فِي الصَّلاَةِ قَالَ وَمَا ذَاكَ قَالَ صَلَّيْتَ خَمْسًا فَسَجَدَ سَجْدَتَيْنِ بَعْدَ مَا سَلَّمَ  

The Messenger of Allah ﷺ offered prayer. The version of the narrator Ibrahim goes: I do not know whether he increased or decreased (the rak'ahs of prayer). When he gave the salutation, he was asked: Has something new happened in the prayer, Messenger of Allah? He said: What is it? They said: You prayed so many and so many (rak'ahs). He then relented his foot and faced the Qiblah and made two prostrations. He then gave the salutation. When he turned away (finished the prayer), he turned his face to us and said: Had anything new happened in prayer, I would have informed you. I am only a human being and I forget just as you do; so when I forget, remind me, and when any of you is in doubt about his prayer he should aim at what is correct, and complete his prayer in that respect, then give the salutation and afterwards made two prostrations.  

عَبْدُ اللَّهِ صَلَّى رَسُولُ اللَّهِ ﷺ قَالَ إِبْرَاهِيمُ فَلاَ أَدْرِي زَادَ أَمْ نَقَصَ فَلَمَّا سَلَّمَ قِيلَ لَهُ يَا رَسُولَ اللَّهِ أَحَدَثَ فِي الصَّلاَةِ شَىْءٌ قَالَ وَمَا ذَاكَ قَالُوا صَلَّيْتَ كَذَا وَكَذَا فَثَنَى رِجْلَهُ وَاسْتَقْبَلَ الْقِبْلَةَ فَسَجَدَ بِهِمْ سَجْدَتَيْنِ ثُمَّ سَلَّمَ فَلَمَّا انْفَتَلَ أَقْبَلَ عَلَيْنَا بِوَجْهِهِ ﷺ فَقَالَ إِنَّهُ لَوْ حَدَثَ فِي الصَّلاَةِ شَىْءٌ أَنْبَأْتُكُمْ بِهِ وَلَكِنْ إِنَّمَا أَنَا بَشَرٌ أَنْسَى كَمَا تَنْسَوْنَ فَإِذَا نَسِيتُ فَذَكِّرُونِي وَقَالَ إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلْيَتَحَرَّ الصَّوَابَ فَلْيُتِمَّ عَلَيْهِ ثُمَّ لْيُسَلِّمْ ثُمَّ لْيَسْجُدْ سَجْدَتَيْنِ  

The narrator Husain also reported it like al-A'mash.  

فَإِذَا نَسِيَ أَحَدُكُمْ فَلْيَسْجُدْ سَجْدَتَيْنِ ثُمَّ تَحَوَّلَ فَسَجَدَ سَجْدَتَيْنِ  

Messenger of Allah, has (the number of the rak’ahs of) the prayer been increased ? he said : No. they said; you have offered five rak’ahs of prayer. He then turned away and performed two prostrations, and afterwards gave the salutation. He then said : I am only a human being, I forget, as you forget.  

عَبْدُ اللَّهِ صَلَّى بِنَا رَسُولُ اللَّهِ ﷺ خَمْسًا فَلَمَّا انْفَتَلَ تَوَشْوَشَ الْقَوْمُ بَيْنَهُمْ فَقَالَ مَا شَأْنُكُمْ قَالُوا يَا رَسُولَ اللَّهِ هَلْ زِيدَ فِي الصَّلاَةِ قَالَ لاَ قَالُوا فَإِنَّكَ صَلَّيْتَ خَمْسًا فَانْفَتَلَ فَسَجَدَ سَجْدَتَيْنِ ثُمَّ سَلَّمَ ثُمَّ قَالَ إِنَّمَا أَنَا بَشَرٌ أَنْسَى كَمَا تَنْسَوْنَ  

One day the Messenger of Allah ﷺ prayed and gave the salutation while a rak'ah of the prayer remained to be offered. A man went to him and said: You forgot to offer one rak'ah of prayer. Then he returned and entered the mosque and ordered Bilal (to utter the Iqamah). He uttered the Iqamah for prayer. He then led the people in one rak'ah of prayer. I stated it to the people. They asked me: Do you know who he was? I said: No, but I can recognise him if I see him. Then the man passed by me, I said: It is he. The people said: This is Talhah ibn Ubaydullah.  

أَنَّ رَسُولَ اللَّهِ ﷺ صَلَّى يَوْمًا فَسَلَّمَ وَقَدْ بَقِيَتْ مِنَ الصَّلاَةِ رَكْعَةٌ فَأَدْرَكَهُ رَجُلٌ فَقَالَ نَسِيتَ مِنَ الصَّلاَةِ رَكْعَةً فَرَجَعَ فَدَخَلَ الْمَسْجِدَ وَأَمَرَ بِلاَلاً فَأَقَامَ الصَّلاَةَ فَصَلَّى لِلنَّاسِ رَكْعَةً فَأَخْبَرْتُ بِذَلِكَ النَّاسَ فَقَالُوا لِي أَتَعْرِفُ الرَّجُلَ قُلْتُ لاَ إِلاَّ أَنْ أَرَاهُ فَمَرَّ بِي فَقُلْتُ هَذَا هُوَ فَقَالُوا هَذَا طَلْحَةُ بْنُ عُبَيْدِ اللَّهِ