Musnad Ahmad 27:1902

Ya'qub narrated to us, my father narrated to us, from Salih. Ibn Shihab said: Salim ibn Abdullah informed me that Abdullah ibn Umar said: The Messenger of Allah (peace and blessings of Allah be upon him) set out, and with him was a group of his companions, among whom was Umar ibn al-Khattab, until he found Ibn Sayyad, a boy who had neared the age of puberty, playing with the boys near the fortress of Banu Mu'awiyah. He then mentioned the rest of its meaning.