Musnad Ahmad 27:1320

'Affan narrated to us, Hammad ibn Salamah narrated to us, Ayyub informed us, from Nafi', from Ibn 'Umar, that the Messenger of Allah (ﷺ) "forbade al-qaza'." Hammad said: "Its explanation is that some of a boy's head is shaved, and a tuft of hair is left on it."