Muwatta Malik: Book 5, Hadith 8

Yahya related to me from Malik from Abu’n Nadr, the mawla of Umar ibn Ubaydullah, from Malik ibn Abi Amir that Uthman ibn Affan used to say in khutbas, and he would seldom omit it if he was giving the khutba, “When the imam stands delivering the khutba on the day of jumua, listen and pay attention, for there is the same portion for someone who pays attention but cannot hear as for someone who pays attention and hears. And when the iqama of the prayer is called, straighten your rows and make your shoulders adjacent to each other, because the straightening of the rows is part of the completion of the prayer.” Then he would not say the takbir until some men who had been entrusted with straightening the rows came and told him that they were straight. Then he would say the takbir.