Musannaf Ibn Abi Shayba 3:1155

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from Malik bin Anas, from Salim Abu al-Nadr, from Malik bin Abi Amir, he said: I heard Uthman while he was saying: "Straighten yourselves and align the shoulders, for indeed, from the perfection of the prayer is the establishing of the rows." He said: And he would not say the Takbir until men whom he had appointed to establish the rows came to him.