Musnad Ahmad 27:726

Yahya narrated to us, from Ubaydullah, [who said]: Umar ibn Nafi’ informed me, from his father, from Ibn Umar, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade *al-qaza’*." I said: "And what is *al-qaza’*?" He said: "That the head of a young boy be shaved and some of it be left."