Musannaf Ibn Abi Shayba 3:1723

Al-Hasan narrated to us, he said: Baqi narrated to us, he said: Abu Bakr Abdullah bin Muhammad bin Abi Shaybah narrated to us, he said: Sufyan bin Uyaynah narrated to us, he said: Abdullah bin Abi Yazid saw Husayn bin Ali at the basin of Zamzam while the prayer had been established. Something was being disputed between the Imam and some of the people, and the caller called out, "The prayer has been established." They began saying to him, "Sit down," but he would say, "The prayer has been established."