Congregational Prayer and Its Virtue
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 57
وَ قَدْ رُوِيَ: «أَنَّهُ إِنْ خَافَ عَلَى نَفْسِهِ مِنْ أَجْلِ مَنْ يُصَلِّي مَعَهُ صَلَّى اَلرَّكْعَتَيْنِ اَلْأَخِيرَتَيْنِ وَ جَعَلَهُمَا تَطَوُّعاً.
It has been narrated: "If a person fears for himself due to those praying with him, he should pray the last two rak'ahs and make them voluntary (nafl) prayers”.
- Ṣaḥīḥ al-Faqīh