عَنْ إِبْرَاهِيمَ عَنْ عَلْقَمَةَ عَنْ عَبْدِ اللَّهِ (بْنِ مَسْعُودٍ) رَضِيَ اللَّهُ عَنْهُ قَالَ: صَلَّى رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ صَلَاةً فَلَا أَدْرِي زَادَ أَمْ نَقَصَ فَلَمَّا سَلَّمَ، قِيلَ لَهُ: يَا رَسُولَ اللَّهِ! هَلْ حَدَثَ فِي الصَّلَاةِ شَيْءٌ؟ قَالَ: ((لَا، وَمَا ذَٰكَ؟)) قَالُوا: صَلَّيْتَ كَذَا وَكَذَا، قَالَ: فَثَنَى رِجْلَيْهِ فَسَجَدَ سَجْدَتَيِ السَّهْوِ، فَلَمَّا سَلَّمَ، قَالَ: ((إِنَّمَا أَنَا بَشَرٌ أَنْسَى كَمَا تَنْسَوْنَ، وَإِذَا شَكَّ أَحَدُكُمْ فِي الصَّلَاةِ فَلْيَتَحَرَّ الصَّلَاةَ، فَإِذَا سَلَّمَ فَلْيَسْجُدْ سَجْدَتَيْنِ))
It is narrated from Sayyiduna Abdullah bin Mas’ud (may Allah be pleased with him), he says: The Messenger of Allah (peace and blessings be upon him) led a prayer, I do not remember whether there was an addition or a deficiency. When the Prophet (peace and blessings be upon him) finished the prayer with salam, someone said: O Messenger of Allah! Has something new been prescribed in the prayer? He (peace and blessings be upon him) said: (No, and what is it (that you are sensing)? The people said: You led such and such number of units in the prayer. So the Prophet (peace and blessings be upon him) folded his legs and performed two prostrations of forgetfulness (sahw), then when he finished with salam, he said: I am only a human being, just as you forget, I also forget. When any of you is in doubt during the prayer, he should seek (the correct number of units in the prayer), and when he finishes with salam, he should perform two prostrations.