Congregational Prayer and Its Virtue
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 57
وَ سَأَلَ عَمَّارٌ اَلسَّابَاطِيُّ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ: عَنْ رَجُلٍ سَهَا خَلْفَ إِمَامٍ بَعْدَ مَا اِفْتَتَحَ اَلصَّلاَةَ فَلَمْ يَقُلْ شَيْئاً وَ لَمْ يُكَبِّرْ وَ لَمْ يُسَبِّحْ وَ لَمْ يَتَشَهَّدْ حَتَّى يُسَلِّمَ فَقَالَ «قَدْ جَازَتْ صَلاَتُهُ وَ لَيْسَ عَلَيْهِ شَيْءٌ إِذَا سَهَا خَلْفَ اَلْإِمَامِ وَ لاَ سَجْدَتَا اَلسَّهْوِ لِأَنَّ اَلْإِمَامَ ضَامِنٌ لِصَلاَةِ مَنْ صَلَّى خَلْفَهُ.
Ammar al-Sabati asked Imam Jafar ibn Muhammad Al-Sadiq عليه السلام about a man who becomes forgetful while praying behind an Imam after beginning the prayer. He neither said anything, made the takbir (saying 'Allahu Akbar'), glorified (did tasbih), nor recited the tashahhud until the Imam concluded the prayer with the taslim. Imam عليه السلام said: "His prayer is valid, and there is no obligation upon him. If he was forgetful while praying behind the Imam, he is not required to perform the two prostrations for forgetfulness because the Imam is responsible for the prayer of those who pray behind him”.
- Ṣaḥīḥ al-Faqīh, p. 58