Messenger of Allah ﷺ said, "When the call for prayer is made, Satan takes to his heels passing wind so that he may not hear the Adhan and when the call is finished he comes back, and when the Iqama is pronounced, Satan again takes to his heels, and when the Iqama is finished he comes back again and tries to interfere with the person and his thoughts and say, "Remember this and that (which he has not thought of before the prayer)", till the praying person forgets how much he has prayed. If anyone of you does not remember whether he has offered three or four rakʿat then he should perform two prostrations of Sahu while sitting.  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا نُودِيَ بِالصَّلاَةِ أَدْبَرَ الشَّيْطَانُ وَلَهُ ضُرَاطٌ حَتَّى لاَ يَسْمَعَ الأَذَانَ فَإِذَا قُضِيَ الأَذَانُ أَقْبَلَ فَإِذَا ثُوِّبَ بِهَا أَدْبَرَ فَإِذَا قُضِيَ التَّثْوِيبُ أَقْبَلَ حَتَّى يَخْطِرَ بَيْنَ الْمَرْءِ وَنَفْسِهِ يَقُولُ اذْكُرْ كَذَا وَكَذَا مَا لَمْ يَكُنْ يَذْكُرُ حَتَّى يَظَلَّ الرَّجُلُ إِنْ يَدْرِي كَمْ صَلَّى فَإِذَا لَمْ يَدْرِ أَحَدُكُمْ كَمْ صَلَّى ثَلاَثًا أَوْ أَرْبَعًا فَلْيَسْجُدْ سَجْدَتَيْنِ وَهْوَ جَالِسٌ  

Messenger of Allah ﷺ once led us in a prayer and offered two rakʿat and got up (for the third rakʿa) without sitting (after the second rakʿa). The people also got up with him, and when he was about to finish his prayer, we waited for him to finish the prayer with Taslim but he said Takbir before Taslim and performed two prostrations while sitting and then finished the prayer with Taslim.  

أَنَّهُ قَالَ صَلَّى لَنَا رَسُولُ اللَّهِ ﷺ رَكْعَتَيْنِ مِنْ بَعْضِ الصَّلَوَاتِ ثُمَّ قَامَ فَلَمْ يَجْلِسْ فَقَامَ النَّاسُ مَعَهُ فَلَمَّا قَضَى صَلاَتَهُ وَنَظَرْنَا تَسْلِيمَهُ كَبَّرَ قَبْلَ التَّسْلِيمِ فَسَجَدَ سَجْدَتَيْنِ وَهُوَ جَالِسٌ ثُمَّ سَلَّمَ  

The Prophet ﷺ prayed (and the sub-narrator Ibrahim said, "I do not know whether he prayed more or less than usual"), and when he had finished the prayers he was asked, "O Messenger of Allah ﷺ! Has there been any change in the prayers?" He said, "What is it?' The people said, "You have prayed so much and so much." So the Prophet ﷺ bent his legs, faced the Qibla and performed two prostration's (of Sahu) and finished his prayers with Taslim (by turning his face to right and left saying: 'As-Salamu ʿAlaikum- Warahmat-ullah'). When he turned his face to us he said, "If there had been anything changed in the prayer, surely I would have informed you but I am a human being like you and liable to forget like you. So if I forget remind me and if anyone of you is doubtful about his prayer, he should follow what he thinks to be correct and complete his prayer accordingly and finish it and do two prostrations (of Sahu).  

عَبْدُ اللَّهِ صَلَّى النَّبِيُّ ﷺ قَالَ إِبْرَاهِيمُ لاَ أَدْرِي زَادَ أَوْ نَقَصَ فَلَمَّا سَلَّمَ قِيلَ لَهُ يَا رَسُولَ اللَّهِ أَحَدَثَ فِي الصَّلاَةِ شَىْءٌ قَالَ وَمَا ذَاكَ قَالُوا صَلَّيْتَ كَذَا وَكَذَا فَثَنَى رِجْلَيْهِ وَاسْتَقْبَلَ الْقِبْلَةَ وَسَجَدَ سَجْدَتَيْنِ ثُمَّ سَلَّمَ فَلَمَّا أَقْبَلَ عَلَيْنَا بِوَجْهِهِ قَالَ إِنَّهُ لَوْ حَدَثَ فِي الصَّلاَةِ شَىْءٌ لَنَبَّأْتُكُمْ بِهِ وَلَكِنْ إِنَّمَا أَنَا بَشَرٌ مِثْلُكُمْ أَنْسَى كَمَا تَنْسَوْنَ فَإِذَا نَسِيتُ فَذَكِّرُونِي وَإِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلْيَتَحَرَّى الصَّوَابَ فَلْيُتِمَّ عَلَيْهِ ثُمَّ يُسَلِّمْ ثُمَّ يَسْجُدْ سَجْدَتَيْنِ  

The Prophet ﷺ led us in the Zuhr prayer, offering only two rakʿat and then (finished it) with Taslim, and went to a piece of wood in front of the mosque and put his hand over it. Abu Bakr and ʿUmar were also present among the people on that day but dared not talk to him (about his unfinished prayer). And the hasty people went away, wondering. "Has the prayer been shortened" Among the people there was a man whom the Prophet ﷺ used to call Dhul-Yadain (the longarmed). He said, "O Allah's Prophet! Have you forgotten or has the prayer been shortened?" The Prophet ﷺ said, "Neither have I forgotten, nor has it been shortened." They (the people) said, "Surely, you have forgotten, O Messenger of Allah ﷺ!" The Prophet ﷺ said, Dhul-Yadain has told the truth." So the Prophet ﷺ got up and offered other two rakʿat and finished his prayer with Taslim. Then he said Takbir, performed a prostration of ordinary duration or longer, then he raised his head and said Takbir and performed another prostration of ordinary duration or longer and then raised his head and said Takbir (i.e. he performed the two prostrations of Sahu, i.e., forgetfulness).  

عَنْ أَبِي هُرَيْرَةَ صَلَّى بِنَا النَّبِيُّ ﷺ الظُّهْرَ رَكْعَتَيْنِ ثُمَّ سَلَّمَ ثُمَّ قَامَ إِلَى خَشَبَةٍ فِي مُقَدَّمِ الْمَسْجِدِ وَوَضَعَ يَدَهُ عَلَيْهَا وَفِي الْقَوْمِ يَوْمَئِذٍ أَبُو بَكْرٍ وَعُمَرُ فَهَابَا أَنْ يُكَلِّمَاهُ وَخَرَجَ سَرَعَانُ النَّاسِ فَقَالُوا قَصُرَتِ الصَّلاَةُ وَفِي الْقَوْمِ رَجُلٌ كَانَ النَّبِيُّ ﷺ يَدْعُوهُ ذَا الْيَدَيْنِ فَقَالَ يَا نَبِيَّ اللَّهِ أَنَسِيتَ أَمْ قَصُرَتْ فَقَالَ لَمْ أَنْسَ وَلَمْ تَقْصُرْ قَالُوا بَلْ نَسِيتَ يَا رَسُولَ اللَّهِ قَالَ صَدَقَ ذُو الْيَدَيْنِ فَقَامَ فَصَلَّى رَكْعَتَيْنِ ثُمَّ سَلَّمَ ثُمَّ كَبَّرَ فَسَجَدَ مِثْلَ سُجُودِهِ أَوْ أَطْوَلَ ثُمَّ رَفَعَ رَأْسَهُ وَكَبَّرَ ثُمَّ وَضَعَ مِثْلَ سُجُودِهِ أَوْ أَطْوَلَ ثُمَّ رَفَعَ رَأْسَهُ وَكَبَّرَ