Sunan an-Nasa'i, Book 13, Hadith 69 (In-Book) ⓘ
It was narrated that Ibrahim said: "They used to say: 'If one is not sure of what he estimates is correct, then prostrate twice.'"
أَخْبَرَنَا سُوَيْدُ بْنُ نَصْرٍ، قَالَ أَنْبَأَنَا عَبْدُ اللَّهِ، عَنِ ابْنِ عَوْنٍ، عَنْ إِبْرَاهِيمَ، قَالَ كَانُوا يَقُولُونَ إِذَا أَوْهَمَ يَتَحَرَّى الصَّوَابَ ثُمَّ يَسْجُدُ سَجْدَتَيْنِ .
also referenced under: Hadith 1247 (Reference) & Volume 2, Book 13, Hadith 1248 (DarusSalam)