Musnad Ahmad 27:1752

Yahya ibn Abi Bukayr narrated to us, Zuhayr narrated to us, Umar ibn Nafi’ narrated to us, from his father, from Abdullah ibn Umar, who said: "The Messenger of Allah, may Allah bless him and grant him peace, forbade *al-qaza’*." And *al-qaza’* is that a boy’s head is shaved, and some of his hair is left.