(25) باب التَّحَرِّي
It was narrated that 'Abdullah said:
"Whoever us not sure about his prayer, then let him estimate what is correct, then let him prostrate twice after he finished his prayer, while he is sitting."
Reference: Sunan an-Nasa'i 1245
In-book reference: Book 13, Hadith 67
English translation: Vol. 2, Book 13, Hadith 1246