by Imam Malik, translation by A'isha `Abdarahman at-Tarjumana and Ya`qub Johnson , Volume: Shortening the Prayer
Chapter: Straightening the Rows
Yahya related to me from Malik from Nafi that Umar ibn al-Khattab used to order the rows to be straightened, and when they had come to him and told him that the rows were straight he would say the takbir.
Yahya related to me from Malik from his paternal uncle, Abu Suhayl ibn Malik, that his father said, "I was with Uthman ibn Affan when the iqama was said for the prayer and I was talking to him about being assigned a definite allowance by him. I continued talking to him while he was levelling some small stones with his sandals, and then some men that he had entrusted to straighten the rows came and told him that the rows were straight. He said to me, 'Line up in the row,' and then he said the takbir."