(25) باب التَّحَرِّي
It was narrated that Ibrahim said:
"They used to say: 'If one is not sure of what he estimates is correct, then prostrate twice.'"
Reference: Sunan an-Nasa'i 1247
In-book reference: Book 13, Hadith 69
English translation: Vol. 2, Book 13, Hadith 1248