Sunan of Abu Dawood Book 2, Hadith 1024

حَدَّثَنَا مُحَمَّدُ بْنُ الْعَلاَءِ، حَدَّثَنَا أَبُو خَالِدٍ، عَنِ ابْنِ عَجْلاَنَ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ عَطَاءِ بْنِ يَسَارٍ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلْيُلْقِ الشَّكَّ وَلْيَبْنِ عَلَى الْيَقِينِ فَإِذَا اسْتَيْقَنَ التَّمَامَ سَجَدَ سَجْدَتَيْنِ فَإِنْ كَانَتْ صَلاَتُهُ تَامَّةً كَانَتِ الرَّكْعَةُ نَافِلَةً وَالسَّجْدَتَانِ وَإِنْ كَانَتْ نَاقِصَةً كَانَتِ الرَّكْعَةُ تَمَامًا لِصَلاَتِهِ وَكَانَتِ السَّجْدَتَانِ مُرْغِمَتَىِ الشَّيْطَانِ ‏"‏ ‏.‏ قَالَ أَبُو دَاوُدَ رَوَاهُ هِشَامُ بْنُ سَعْدٍ وَمُحَمَّدُ بْنُ مُطَرِّفٍ عَنْ زَيْدٍ عَنْ عَطَاءِ بْنِ يَسَارٍ عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ عَنِ النَّبِيِّ صلى الله عليه وسلم وَحَدِيثُ أَبِي خَالِدٍ أَشْبَعُ ‏.‏

‘Ata' b. Yasar said that Abu Sa'id al-Khudri reported the Messenger of Allah (ﷺ) as saying:when one of you is in doubt about his prayer (i.e, how much he has prayed), he should throw away his doubt and base his prayer on what he is sure of. When he is sure about the completion of his prayer, he should make two prostrations (at the end of the prayer). If the prayer is complete, the additional rak'ah and the two prostrations will be supererogatory prayer. If the prayer is incomplete, the additional rak'ahs will compensate it, and the two prostrations will be a disgrace for the devil