Musannaf Ibn Abi Shaybah

Hazrat Abu Mas'ud narrates that the Messenger of Allah, peace and blessings be upon him, would touch our shoulders with his hand during prayer to straighten the rows and would say: "Keep the rows straight, do not stand ahead or behind, otherwise Allah the Exalted will create discord among your hearts." Those among you who are possessed of intellect and understanding should stand close to me in prayer, and after them, those who are lesser than them in understanding should stand. Hazrat Abu Mas'ud says that today, your great differences are due to not straightening the rows.