Sahih al-Bukhari 6671
Sahih al-Bukhari · Book 83: Oaths and Vows · If someone does something against his oath due to forgetfulness
Narrated Ibn Mas`ud:
حَدَّثَنِي إِسْحَاقُ بْنُ إِبْرَاهِيمَ ، سَمِعَ عَبْدَ الْعَزِيزِ بْنَ عَبْدِ الصَّمَدِ ، حَدَّثَنَا مَنْصُورٌ ، عَنْ إِبْرَاهِيمَ ، عَنْ عَلْقَمَةَ ، عَنِ ابْنِ مَسْعُودٍ ـ رضى الله عنه أَنَّ نَبِيَّ اللَّهِ صلى الله عليه وسلم صَلَّى بِهِمْ صَلاَةَ الظُّهْرِ، فَزَادَ أَوْ نَقَصَ مِنْهَا ـ قَالَ مَنْصُورٌ لاَ أَدْرِي إِبْرَاهِيمُ وَهِمَ أَمْ عَلْقَمَةُ ـ قَالَ قِيلَ يَا رَسُولَ اللَّهِ أَقَصُرَتِ الصَّلاَةُ أَمْ نَسِيتَ قَالَ " وَمَا ذَاكَ ". قَالُوا صَلَّيْتَ كَذَا وَكَذَا. قَالَ فَسَجَدَ بِهِمْ سَجْدَتَيْنِ ثُمَّ قَالَ " هَاتَانِ السَّجْدَتَانِ لِمَنْ لاَ يَدْرِي، زَادَ فِي صَلاَتِهِ أَمْ نَقَصَ، فَيَتَحَرَّى الصَّوَابَ، فَيُتِمُّ مَا بَقِيَ، ثُمَّ يَسْجُدُ سَجْدَتَيْنِ " .
that Allah's Prophet led them in the Zuhr prayer and he offered either more or less rak`at, and it was said to him, "O Allah's Messenger (ﷺ) ! Has the prayer been reduced, or have you forgotten?" He asked, "What is that?" They said, "You have prayed so many rak`at." So he performed with them two more prostrations and said, "These two prostrations are to be performed by the person who does not know whether he has prayed more or less (rak`at) in which case he should seek to follow what is right. And then complete the rest (of the prayer) and perform two extra prostrations."