The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ رَوَى عَلِيُّ بْنُ نُعْمَانَ اَلرَّازِيُّ أَنَّهُ قَالَ: كُنْتُ مَعَ أَصْحَابٍ لِي فِي سَفَرٍ وَ أَنَا إِمَامُهُمْ فَصَلَّيْتُ بِهِمُ اَلْمَغْرِبَ فَسَلَّمْتُ فِي اَلرَّكْعَتَيْنِ اَلْأَوَّلَتَيْنِ فَقَالَ أَصْحَابِي إِنَّمَا صَلَّيْتَ بِنَا رَكْعَتَيْنِ فَكَلَّمْتُهُمْ وَ كَلَّمُونِي فَقَالُوا أَمَّا نَحْنُ فَنُعِيدُ فَقُلْتُ لَكِنِّي لاَ أُعِيدُ وَ أُتِمُّ بِرَكْعَةٍ فَأَتْمَمْتُ بِرَكْعَةٍ ثُمَّ سِرْنَا وَ أَتَيْتُ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ وَ ذَكَرْتُ لَهُ اَلَّذِي كَانَ مِنْ أَمْرِنَا فَقَالَ لِي «أَنْتَ أَصْوَبُ مِنْهُمْ فِعْلاً إِنَّمَا يُعِيدُ مَنْ لاَ يَدْرِي مَا صَلَّى.
Ali ibn Nu'man al-Razi narrated that He said: I was traveling with some companions, and I was their Imam (leading the prayer). I led them in Maghrib prayer and mistakenly ended the prayer after two units (rak'ahs). My companions told me that I had only led them in two rak'ahs. We discussed it among ourselves, and they decided that they would repeat their prayer. I, however, decided not to repeat it but to complete it by adding one more rak'ah. So, I completed it with an additional rak'ah, and we then continued our journey. Later, I went to Imam Ja'far Al-Sadiq عليه السلام and told him about what had happened. Imam عليه السلام said to me: "You acted more correctly than they did. Only the one who does not know what he has prayed needs to repeat (the prayer)”.
- Ṣaḥīḥ al-Faqīh