The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ رَوَى اَلْعَلاَءُ عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ عَنْ أَبِي جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ: فِي رَجُلٍ شَكَّ بَعْدَ مَا سَجَدَ أَنَّهُ لَمْ يَرْكَعْ فَقَالَ «يَمْضِي فِي صَلاَتِهِ حَتَّى يَسْتَيْقِنَ أَنَّهُ لَمْ يَرْكَعْ فَإِنِ اِسْتَيْقَنَ أَنَّهُ لَمْ يَرْكَعْ فَلْيُلْقِ اَلسَّجْدَتَيْنِ اَللَّتَيْنِ لاَ رُكُوعَ لَهُمَا وَ يَبْنِي عَلَى صَلاَتِهِ اَلَّتِي عَلَى اَلتَّمَامِ فَإِنْ كَانَ لَمْ يَسْتَيْقِنْ إِلاَّ مِنْ بَعْدِ مَا فَرَغَ وَ اِنْصَرَفَ فَلْيَقُمْ وَ لْيُصَلِّ رَكْعَةً وَ سَجْدَتَيْنِ وَ لاَ شَيْءَ عَلَيْهِ.
Al-‘Ala' narrated from Muhammad ibn Muslim from Imam Muhammad ibn Ali Al-Baqir عليه السلام that He said: "If a man doubts after prostrating that he has not bowed (rak'a), he should continue with his prayer until he is certain that he did not bow. If he becomes certain that he did not bow, then he should perform the two prostrations (sajdatayn) that have no bowing and complete his prayer as though it were perfect. However, if he does not become certain until after he has finished and turned away from the prayer, he should stand up and pray one more rak'ah with two prostrations, and there is no harm upon him."
- Ṣaḥīḥ al-Faqīh