Arabic (Original)
حَدَّثَنَا يُونُسُ بْنُ مُحَمَّدٍ حَدَّثَنَا فُلَيْحٌ عَنْ زَيْدِ بْنِ أَسْلَمَ عَنْ عَطَاءِ بْنِ يَسَارٍ عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلَمْ يَدْرِ كَمْ صَلَّى فَلْيَبْنِ عَلَى الْيَقِينِ حَتَّى إِذَا اسْتَيْقَنَ أَنْ قَدْ أَتَمَّ فَلْيَسْجُدْ سَجْدَتَيْنِ قَبْلَ أَنْ يُسَلِّمَ فَإِنَّهُ إِنْ كَانَتْ صَلَاتُهُ وِتْرًا صَارَتْ شَفْعًا وَإِنْ كَانَتْ شَفْعًا كَانَ ذَلِكَ تَرْغِيمًا لِلشَّيْطَانِ.
English Translation
Yunus ibn Muhammad narrated to us: Fulayh narrated to us, from Zayd ibn Aslam, from Ata' ibn Yasar, from Abu Sa'id al-Khudri, that the Messenger of Allah, peace and blessings be upon him, said: "If one of you is in doubt in his prayer and does not know how many rak'ahs he has prayed, let him proceed upon certainty, and when he is certain that he has completed the prayer, let him prostrate twice before saying the taslim. For if his prayer was odd-numbered, it becomes even, and if it was even, that extra prostration humiliates Satan."