Mishkat al-Masabih 1088
Abu Mas'ud al-Ansari said:
God’s Messenger used to touch our shoulders at the prayer and say, “Keep straight; do not be irregular and so have your hearts irregular. Let those of you who are sedate and prudent be near me, then those who are next to them, then those who are next to them." Abu Mas'ud said, “Nowadays you are much more irregular." 

Muslim transmitted it.
وَعَنْ أَبِي مَسْعُودٍ الْأَنْصَارِيِّ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَمْسَحُ مَنَاكِبَنَا فِي الصَّلَاةِ وَيَقُولُ: «اسْتَوُوا وَلَا تَخْتَلِفُوا فَتَخْتَلِفَ قُلُوبكُمْ ليليني مِنْكُم أولُوا الْأَحْلَامِ وَالنُّهَى ثُمَّ الَّذِينَ يَلُونَهُمْ ثُمَّ الَّذِينَ يَلُونَهُمْ» . قَالَ أَبُو مَسْعُودٍ: فَأَنْتُمُ الْيَوْمَ أَشَدُّ اخْتِلَافا. رَوَاهُ مُسلم