The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ فِي نَوَادِرِ إِبْرَاهِيمَ بْنِ هَاشِمٍ: أَنَّهُ سُئِلَ أَبُو عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ عَنْ إِمَامٍ يُصَلِّي بِأَرْبَعِ نَفَرٍ أَوْ بِخَمْسٍ فَيُسَبِّحُ اِثْنَانِ عَلَى أَنَّهُمْ صَلَّوْا ثَلاَثاً وَ يُسَبِّحُ ثَلاَثَةٌ عَلَى أَنَّهُمْ صَلَّوْا أَرْبَعاً يَقُولُ هَؤُلاَءِ قُومُوا وَ يَقُولُ هَؤُلاَءِ اُقْعُدُوا وَ اَلْإِمَامُ مَائِلٌ مَعَ أَحَدِهِمَا أَوْ مُعْتَدِلُ اَلْوَهْمِ فَمَا يَجِبُ عَلَيْهِمْ قَالَ «لَيْسَ عَلَى اَلْإِمَامِ سَهْوٌ إِذَا حَفِظَ عَلَيْهِ مَنْ خَلْفَهُ سَهْوَهُ بِاتِّفَاقٍ مِنْهُمْ وَ لَيْسَ عَلَى مَنْ خَلْفَ اَلْإِمَامِ سَهْوٌ إِذَا لَمْ يَسْهُ اَلْإِمَامُ وَ لاَ سَهْوَ فِي سَهْوٍ وَ لَيْسَ فِي اَلْمَغْرِبِ سَهْوٌ وَ لاَ فِي اَلْفَجْرِ سَهْوٌ وَ لاَ فِي اَلرَّكْعَتَيْنِ اَلْأَوَّلَتَيْنِ مِنْ كُلِّ صَلاَةٍ سَهْوٌ، فَإِذَا اِخْتَلَفَ عَلَى اَلْإِمَامِ مَنْ خَلْفَهُ فَعَلَيْهِ وَ عَلَيْهِمْ فِي اَلاِحْتِيَاطِ وَ اَلْإِعَادَةِ وَ اَلْأَخْذِ بِالْجَزْمِ.
In the Nawaadir of Ibrahim ibn Hashim, it is narrated that Imam Jafar ibn Muhammad Al-Sadiq عليه السلام was asked about an Imam leading a prayer with four or five people, and two of them testify that they prayed three rak'ahs, while three testify that they prayed four rak'ahs. Some say, "Stand up," and others say, "Sit down," and the Imam is unsure, either leaning towards one or being uncertain. What should be done? Imam عليه السلام said: "There is no fault on the Imam if those behind him agree that he made a mistake. There is no fault on those behind the Imam if the Imam has not erred. There is no confusion in the Maghrib prayer, nor in Fajr, nor in the first two rak'ahs of any prayer. If there is disagreement among those behind the Imam, then both the Imam and the congregation should take precaution and repeat the prayer, acting decisively."
- Ṣaḥīḥ al-Faqīh