Congregational Prayer and Its Virtue
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 57
وَ رَوَى عَبْدُ اَللَّهِ بْنُ اَلْمُغِيرَةِ قَالَ كَانَ مَنْصُورُ بْنُ حَازِمٍ يَقُولُ: «إِذَا أَتَيْتَ اَلْإِمَامَ وَ هُوَ جَالِسٌ قَدْ صَلَّى رَكْعَتَيْنِ فَكَبِّرْ ثُمَّ اِجْلِسْ فَإِذَا قُمْتَ فَكَبِّرْ.
Abdullah ibn al-Mughira narrated that Mansur ibn Hazim used to say: "If you come to the Imam and he is sitting, having already prayed two rak'ahs, say the takbir (Allahu Akbar) and then sit. When you stand up, say the takbir again."
- Ṣaḥīḥ al-Faqīh