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، 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."
Reference : Sunan an-Nasa'i 1246In-book reference : Book 13, Hadith 68English translation : Vol. 2, Book 13, Hadith 1247