سُنَنُ النَّسَائِيِّ
Sunan an-Nasa’i 1246
The Book of Forgetfulness (In Prayer)
Estimating (what is most likely the case)
أَخْبَرَنَا سُوَيْدُ بْنُ نَصْرٍ، قَالَ أَنْبَأَنَا عَبْدُ اللَّهِ، عَنْ مِسْعَرٍ، عَنِ الْحَكَمِ، عَنْ أَبِي وَائِلٍ، عَنْ عَبْدِ اللَّهِ، قَالَ مَنْ شَكَّ أَوْ أَوْهَمَ فَلْيَتَحَرَّ الصَّوَابَ ثُمَّ لْيَسْجُدْ سَجْدَتَيْنِ .
It was narrated that 'Abdullah said: "Whoever has doubt, or is not sure, let him estimate what is correct, then let him prostrate twice."
Sunan an-Nasa'i, 1246Sahih (Darussalam)