Musnad Ahmad 2:180

Aswad ibn Amir narrated to us, Hammad ibn Salamah narrated to us, from Abu Sinan, from Ubayd ibn Adam, Abu Maryam, and Abu Shu’ayb: That Umar ibn al-Khattab was at al-Jabiyah... and he mentioned the conquest of Bayt al-Maqdis (Jerusalem). Abu Salamah said: Abu Sinan narrated to me, from Ubayd ibn Adam, who said: I heard Umar ibn al-Khattab say to Ka’b: "Where do you think I should pray?" Ka’b said: "If you take my advice, you will pray behind the Rock, so that the entire city of Jerusalem is before you." Umar said: "You have imitated Judaism. No, but I will pray where the Messenger of Allah (peace and blessings of Allah be upon him) prayed." He then stepped forward toward the Qibla and prayed. Then he came and spread out his cloak, and he swept the rubbish into his cloak, and the people swept [as well].