Sunan An-Nasa'i
Hadith #1251
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