Congregational Prayer and Its Virtue
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 57
وَ رَوَى هِشَامُ بْنُ سَالِمٍ عَنْهُ عَلَيْهِ اَلسَّلاَمُ أَنَّهُ قَالَ: فِي اَلرَّجُلِ يُصَلِّي اَلصَّلاَةَ وَحْدَهُ ثُمَّ يَجِدُ جَمَاعَةً قَالَ «يُصَلِّي مَعَهُمْ وَ يَجْعَلُهَا اَلْفَرِيضَةَ إِنْ شَاءَ.
Hisham ibn Salim narrated from him عليه السلام that He said regarding a man who prays alone and then finds a congregation: "He may pray with them and count it as his obligatory (fardh) prayer if he wishes”.
- Ṣaḥīḥ al-Faqīh