Musannaf Ibn Abi Shayba 34:254

Al-Sahmi narrated to us, Hamid narrated to us, he said: I asked Al-Hasan, "Who was the first to deliver the sermon before the prayer?" He said: "Uthman ibn Affan prayed with the people, then he delivered the sermon to them. He saw many people who had not caught the prayer, so they did that."