Musannaf Ibn Abi Shayba 3:3308

Abu Bakr narrated to us, he said: Abda ibn Sulayman narrated to us, from Yahya ibn Sa’id, from Yusuf ibn Abd Allah ibn Salam, who said: The people used to begin with the prayer, then they would follow it with the sermon. When it was the time of Umar and the people became numerous in his era, whenever he went to deliver the sermon, the people would leave barefoot. When Umar saw that, he began with the sermon until he concluded with the prayer.