Musnad Imam Shafi'i

It is narrated from Malik bin Abi Amir that Uthman bin Affan (may Allah be pleased with him) would often say in his Friday sermon: When the Imam stands to deliver the sermon on the day of Jumu'ah, you should remain silent and listen attentively. And the one who remains silent but does not listen attentively will still receive the same reward as the one who remains silent and listens attentively. And when the prayer is established, straighten your rows and align your shoulders with one another, for straightening the rows is part of the completion of the prayer. Then Uthman (may Allah be pleased with him) would not say the opening takbir until those whom he had sent to correct the rows came and informed him that the rows had been straightened. When he received this information, he would then say the opening takbir.