Musnad Ahmad 962:91

'Affan narrated to us, Hammad ibn Salamah narrated to us, Abu Ghalib informed us, from Abu Umamah, that the Messenger of Allah (peace and blessings of Allah be upon him) arrived from Khaybar, and with him were two young boys. 'Ali said: "O Messenger of Allah, will you provide us with a servant?" He said: "Take whichever of them you wish." He said: "Choose for me." He said: "Take this one, and do not strike him, for I have seen him praying while we were coming from Khaybar, and I have been forbidden from striking the people of prayer." He gave the other boy to Abu Dharr and said: "Treat him well." Then he said: "O Abu Dharr, what did the boy do whom I gave to you?" He said: "You commanded me to treat him well, so I freed him."