Congregational Prayer and Its Virtue
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 57
وَ رَوَى اَلْحَلَبِيُّ عَنْهُ عَنْ أَبِيهِ عَلَيْهِمَا اَلسَّلاَمُ قَالَ: «إِذَا صَلَّيْتَ صَلاَةً وَ أَنْتَ فِي اَلْمَسْجِدِ فَأُقِيمَتِ اَلصَّلاَةُ فَإِنْ شِئْتَ فَاخْرُجْ وَ إِنْ شِئْتَ فَصَلِّ مَعَهُمْ وَ اِجْعَلْهَا تَسْبِيحاً.
Al-Halabi narrated from the Imam عليه السلام from his father عليه السلام who said: "If you have already prayed and you are in the mosque, and the congregational prayer is established, then if you wish, you may leave, or if you wish, you may pray with them and consider it as a form of tasbih (remembrance and glorification)."
- Ṣaḥīḥ al-Faqīh, p. 59