Sunan an-Nasa'i 1245
It was narrated that 'Abdullah said:
"Whoever us not sure about his prayer, then let him estimate what is correct, then let him prostrate twice after he finished his prayer, while he is sitting."
أَخْبَرَنَا سُوَيْدُ بْنُ نَصْرٍ، قَالَ أَنْبَأَنَا عَبْدُ اللَّهِ، عَنْ شُعْبَةَ، عَنِ الْحَكَمِ، قَالَ سَمِعْتُ أَبَا وَائِلٍ، يَقُولُ قَالَ عَبْدُ اللَّهِ مَنْ أَوْهَمَ فِي صَلاَتِهِ فَلْيَتَحَرَّ الصَّوَابَ ثُمَّ يَسْجُدْ سَجْدَتَيْنِ بَعْدَ مَا يَفْرُغُ وَهُوَ جَالِسٌ ‏.‏