Musannaf Ibn Abi Shayba 3:3723

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from Shu'bah, from Abu 'Imran al-Jawni, from 'Abdullah ibn al-Samit, from Abu Dharr: That an Ethiopian slave arrived at al-Rabadhah, and the prayer was established. He (Abu Dharr) said: "Step forward."