سُنَنُ النَّسَائِيِّ
Sunan an-Nasa’i 1247
The Book of Forgetfulness (In Prayer)
Estimating (what is most likely the case)
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.'"
Sunan an-Nasa'i, 1247Sahih (Darussalam)