ENOfficial
It was narrated that Abdullah bin Ja'far said: "The Messenger of Allah (ﷺ) said: 'Whoever has doubt during his prayer, let him prostrate twice after he has said the taslim'".
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا سُوَيْدُ بْنُ نَصْرٍ، قَالَ أَنْبَأَنَا عَبْدُ اللَّهِ، عَنِ ابْنِ جُرَيْجٍ، قَالَ قَالَ عَبْدُ اللَّهِ بْنُ مُسَافِعٍ عَنْ عُتْبَةَ بْنِ مُحَمَّدِ بْنِ الْحَارِثِ، عَنْ عَبْدِ اللَّهِ بْنِ جَعْفَرٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَنْ شَكَّ فِي صَلاَتِهِ فَلْيَسْجُدْ سَجْدَتَيْنِ بَعْدَ مَا يُسَلِّمُ " .
sunnah.com