Mukhtasar Muslim

Sayyiduna Abu Mas'ud (may Allah be pleased with him) says that for prayer, the Messenger of Allah (peace and blessings be upon him) would pass his hand over our shoulders and say: "Stand straight and do not move forward or backward, otherwise discord will arise in your hearts. Also, let those who are very wise and intelligent stand closest to me, and then those who are next to them." After this, Sayyiduna Abu Mas'ud (may Allah be pleased with him) said: "Today, countless differences have arisen among you people."