Musannaf Ibn Abi Shayba 3:1150

Abu Mu'awiyah and Waki' narrated to us, from al-A'mash, from 'Umarah, from Abu Ma'mar, from Abu Mas'ud, who said: The Prophet, peace and blessings of Allah be upon him, used to touch our shoulders in prayer and say: "Align yourselves and do not differ, lest your hearts differ. Let those among you who are mature and possess understanding be nearest to me, then those who are next to them." Abu Mas'ud said: "So you today are more intense in your disagreement."