Sunan an-Nasa'i, Volume 2, Book 13, Hadith 1251 (DarusSalam) ⓘ
It was narrated from 'Abdullah bin Ja'far that : The Messenger of Allah said: "Whoever has doubt during his prayer, let him prostrate twice after he said the taslim."
أَخْبَرَنَا مُحَمَّدُ بْنُ إِسْمَاعِيلَ بْنِ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا حَجَّاجٌ، قَالَ ابْنُ جُرَيْجٍ أَخْبَرَنِي عَبْدُ اللَّهِ بْنُ مُسَافِعٍ، أَنَّ مُصْعَبَ بْنَ شَيْبَةَ، أَخْبَرَهُ عَنْ عُتْبَةَ بْنِ مُحَمَّدِ بْنِ الْحَارِثِ، عَنْ عَبْدِ اللَّهِ بْنِ جَعْفَرٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ " مَنْ شَكَّ فِي صَلاَتِهِ فَلْيَسْجُدْ سَجْدَتَيْنِ بَعْدَ مَا يُسَلِّمُ " .
also referenced under: Hadith 1250 (Reference) & Book 13, Hadith 72 (In-Book)