Musnad Ahmad 457:40

Waki’ and Abu Mu’awiyah narrated to us, they said: Al-A’mash narrated to us, from ‘Umarah ibn ‘Umayr al-Taymi, from Abu Ma’mar ‘Abdullah ibn Sakhbarah al-Azdi, from Abu Mas’ud al-Ansari, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to touch our shoulders in prayer—Waki’ said—and he would 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, then those who are next to them." Abu Mas’ud said: "So you today are more intense in your differing."