Musnad Ahmad 27:26

Uthman ibn Uthman—meaning al-Ghatafani—narrated to us, Umar ibn Nafi’ informed us, from his father, from Ibn Umar, who said: "The Messenger of Allah (ﷺ) forbade al-qaza’. And al-qaza’ is that a boy’s head is shaved, and some of his hair is left."