Sunan an-Nasai Book 10, Hadith 812

أَخْبَرَنَا بِشْرُ بْنُ خَالِدٍ الْعَسْكَرِيُّ، قَالَ حَدَّثَنَا غُنْدَرٌ، عَنْ شُعْبَةَ، عَنْ سُلَيْمَانَ، عَنْ عُمَارَةَ بْنِ عُمَيْرٍ، عَنْ أَبِي مَعْمَرٍ، عَنْ أَبِي مَسْعُودٍ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَمْسَحُ عَوَاتِقَنَا وَيَقُولُ ‏ "‏ اسْتَوُوا وَلاَ تَخْتَلِفُوا فَتَخْتَلِفَ قُلُوبُكُمْ وَلْيَلِيَنِّي مِنْكُمْ أُولُو الأَحْلاَمِ وَالنُّهَى ثُمَّ الَّذِينَ يَلُونَهُمْ ثُمَّ الَّذِينَ يَلُونَهُمْ ‏"‏ ‏.‏

It was narrated that Abu Mas'ad Al-Ansari said:"The Messenger of Allah (ﷺ) used to gently pat our shoulders (to make sure the row was straight) at the time of prayer, and he would say: 'Keep (the rows) straight; do not differ from one another lest your hearts should suffer from discord. Let those who are mature and wise stand closest to me, then those who are next to them, then those who are next to them