Congregational Prayer and Its Virtue
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 57
وَ رَوَى مُحَمَّدُ بْنُ مُسْلِمٍ عَنْ أَحَدِهِمَا عَلَيْهِمَا اَلسَّلاَمُ: فِي اَلرَّجُلِ يَدْخُلُ اَلْمَسْجِدَ فَيَخَافُ أَنْ تَفُوتَهُ اَلرَّكْعَةُ قَالَ «يَرْكَعُ قَبْلَ أَنْ يَبْلُغَ إِلَى اَلْقَوْمِ وَ يَمْشِي وَ هُوَ رَاكِعٌ حَتَّى يَبْلُغَهُمْ.
Muhammad ibn Muslim narrated from one of the Imams عليه السلام regarding a man who enters the mosque and fears that he will miss the rak'ah (unit of prayer). Imam عليه السلام said: "He should bow (in ruku') before reaching the congregation and then walk while in the bowing position until he joins them."
- Ṣaḥīḥ al-Faqīh, p. 56