Bashr bin Hilal al-Swaf بشر بن هلال الصواف · The Book of Forgetfulness (In Prayer)

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."