Sahih Muslim Book - 5, Hadith - 1272

وَحَدَّثَنِى مُحَمَّدُ بْنُ أَحْمَدَ بْنِ أَبِى خَلَفٍ حَدَّثَنَا مُوسَى بْنُ دَاوُدَ حَدَّثَنَا سُلَيْمَانُ بْنُ بِلاَلٍ عَنْ زَيْدِ بْنِ أَسْلَمَ عَنْ عَطَاءِ بْنِ يَسَارٍ عَنْ أَبِى سَعِيدٍ الْخُدْرِىِّ قَالَ قَالَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- « إِذَا شَكَّ أَحَدُكُمْ فِى صَلاَتِهِ فَلَمْ يَدْرِ كَمْ صَلَّى ثَلاَثًا أَمْ أَرْبَعًا فَلْيَطْرَحِ الشَّكَّ وَلْيَبْنِ عَلَى مَا اسْتَيْقَنَ ثُمَّ يَسْجُدُ سَجْدَتَيْنِ قَبْلَ أَنْ يُسَلِّمَ فَإِنْ كَانَ صَلَّى خَمْسًا شَفَعْنَ لَهُ صَلاَتَهُ وَإِنْ كَانَ صَلَّى إِتْمَامًا لأَرْبَعٍ كَانَتَا تَرْغِيمًا لِلشَّيْطَانِ ».

It was narrated that Abu Sa'eed Al-Khudri said: "The Messenger of Allah (s.a.w) said: 'If one of you is unsure when in prayer and does not know how many (Rak'ah) he has prayed, whether it is three or four, let him ignore what is uncertain and proceed on the basis of what is certain. Then let him prostrate twice before saying the Taslim. Then if he has prayed five (Rak'ah ), that will make his prayer even, and if he has prayed it properly with four, it will annoy the Shaitan."'