It was narrated from 'Abdullah bin Ja'far that:

The Messenger of Allah ﷺ said: "Whoever has doubt during his prayer, let him prostrate twice." (One of the narrators) Hajjaj said: "After he has said the taslim." (Another of them) Rawh said: "While he is sitting."

أَخْبَرَنَا هَارُونُ بْنُ عَبْدِ اللَّهِ، قَالَ حَدَّثَنَا حَجَّاجٌ، وَرَوْحٌ، - هُوَ ابْنُ عُبَادَةَ - عَنِ ابْنِ جُرَيْجٍ، قَالَ أَخْبَرَنِي عَبْدُ اللَّهِ بْنُ مُسَافِعٍ، أَنَّ مُصْعَبَ بْنَ شَيْبَةَ، أَخْبَرَهُ عَنْ عُتْبَةَ بْنِ مُحَمَّدِ بْنِ الْحَارِثِ، عَنْ عَبْدِ اللَّهِ بْنِ جَعْفَرٍ، أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ " مَنْ شَكَّ فِي صَلاَتِهِ فَلْيَسْجُدْ سَجْدَتَيْنِ " . قَالَ حَجَّاجٌ " بَعْدَ مَا يُسَلِّمُ " . وَقَالَ رَوْحٌ " وَهُوَ جَالِسٌ " .