Sunan an-Nasa'i, Hadith 1246 ⓘ
It was narrated that 'Abdullah said: "Whoever has doubt, or is not sure, let him estimate what is correct, then let him prostrate twice."
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 .
also referenced under: Book 13, Hadith 68 (In-Book) & Volume 2, Book 13, Hadith 1247 (DarusSalam)