Hadith 1246
Chapter 25: Estimating (what is most likely the case) - كتاب السهو
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 1246
In-book reference : Book 13, Hadith 68