The Book of Forgetfulness (In Prayer)
13/25. Chapter:
Estimating (what is most likely the case)
Sunan an-Nasai
أَخْبَرَنَا سُوَيْدُ بْنُ نَصْرٍ، قَالَ أَنْبَأَنَا عَبْدُ اللَّهِ، عَنِ ابْنِ عَوْنٍ، عَنْ إِبْرَاهِيمَ، قَالَ كَانُوا يَقُولُونَ إِذَا أَوْهَمَ يَتَحَرَّى الصَّوَابَ ثُمَّ يَسْجُدُ سَجْدَتَيْنِ .
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 1247In-book reference : Book 13, Hadith 69English translation : Vol. 2, Book 13, Hadith 1248