كتاب السهو
Read the English translation first, with the original Arabic directly underneath.
It was narrated from 'Abdullah and attributed to the Prophet (ﷺ):
"If one of you is not sure about his prayer, let him estimate what he thinks is most likely to be correct and complete the prayer on that basis, then let him prostrate twice."
إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلْيَتَحَرَّ الَّذِي يَرَى أَنَّهُ الصَّوَابُ فَيُتِمَّهُ ثُمَّ - يَعْنِي - يَسْجُدُ سَجْدَتَيْنِ