Congregational Prayer and Its Virtue
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 57
وَقَالَ رَجُلٌ لِأَبِي جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ إِنِّي إِمَامُ مَسْجِدِ اَلْحَيِّ فَأَرْكَعُ بِهِمْ وَ أَسْمَعُ خَفَقَانَ نِعَالِهِمْ وَ أَنَا رَاكِعٌ فَقَالَ «اِصْبِرْ رُكُوعَكَ وَ مِثْلَ رُكُوعِكَ فَإِنِ اِنْقَطَعُوا وَ إِلاَّ فَانْتَصِبْ قَائِماً.
A man said to Imam Abu Ja'far عليه السلام, "I am the Imam of the mosque in our neighborhood, and when I bow (in ruku') with them, I hear the sound of their footsteps while I am in ruku'." Imam عليه السلام replied: "Remain in your bowing position for as long as your bowing takes, and if they catch up, (continue); otherwise, stand up straight."
- Ṣaḥīḥ al-Faqīh, p. 55