Musnad Ahmad 962:18

Hassan ibn Musa and ‘Affan narrated to us, they said: Hammad ibn Salamah narrated to us, he said: ‘Affan said: 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. He gifted one of them to ‘Ali ibn Abi Talib and said: "Do not strike him, for I have been forbidden from striking the people of prayer, and I have seen him praying." ‘Affan said in his narration: Abu Ghalib informed us, from Abu Umamah, that the Prophet (peace and blessings of Allah be upon him) arrived from Khaybar, and with him were two young boys. ‘Ali said: "O Messenger of Allah, give us 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 arriving from Khaybar, and I have been forbidden [from striking them]." And he gave Abu Dharr a young boy and said: "Treat him well." Then he emancipated him. The Prophet (peace and blessings of Allah be upon him) said to him: "What did the boy do?" He said: "O Messenger of Allah, you commanded me to treat him well, so I emancipated him."