Sunan Ibn Majah, Volume 2, Book 5, Hadith 1208 (DarusSalam) ⓘ

It was narrated that Mughirah bin Shu’bah said: “The Messenger of Allah said: ‘If anyone of you stands after two Rak’ah, if he has not yet stood up fully, let him sit down again, but if he has stood up fully, then let him not sit down, and let him perform two prostrations for forgetfulness (Sahw).’”

حَدَّثَنَا مُحَمَّدُ بْنُ يَحْيَى، حَدَّثَنَا مُحَمَّدُ بْنُ يُوسُفَ، حَدَّثَنَا سُفْيَانُ، عَنْ جَابِرٍ، عَنِ الْمُغِيرَةِ بْنِ شُبَيْلٍ، عَنْ قَيْسِ بْنِ أَبِي حَازِمٍ، عَنِ الْمُغِيرَةِ بْنِ شُعْبَةَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ ‏ "‏ إِذَا قَامَ أَحَدُكُمْ مِنَ الرَّكْعَتَيْنِ فَلَمْ يَسْتَتِمَّ قَائِمًا فَلْيَجْلِسْ فَإِذَا اسْتَتَمَّ قَائِمًا فَلاَ يَجْلِسْ وَيَسْجُدْ سَجْدَتَىِ السَّهْوِ ‏" ‏ ‏.‏