Musannaf Ibn Abi Shaybah

Hazrat Malik bin Abi Aamir said that that Hazrat Usman used to say: "Stand in straight rows and level your shoulders, for the perfection of the prayer is in the straightening of the rows." Hazrat Usman would not say the Takbeer-e-Tahreema until the person appointed by him to straighten the rows would come and inform him.