Congregational Prayer and Its Virtue
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 57
قَالَ وَقَالَ رَسُولُ اَللَّهِ صَلَّى اَللَّهُ عَلَيْهِ وَ آلِهِ: «أَقِيمُوا صُفُوفَكُمْ فَإِنِّي أَرَاكُمْ مِنْ خَلْفِي كَمَا أَرَاكُمْ مِنْ قُدَّامِي وَ مِنْ بَيْنِ يَدَيَّ وَ لاَ تُخَالِفُوا فَيُخَالِفَ اَللَّهُ بَيْنَ قُلُوبِكُمْ.
The Messenger of Allah صلى الله عليه وعلى آله وسلم said: "Straighten your rows, for I see you from behind me just as I see you from in front of me and from before me. Do not differ, lest Allah سبحانه وتعالى cause discord among your hearts."
- Ṣaḥīḥ al-Faqīh, p. 54