Ibn 'Umar (May Allah be pleased with them) said:
The Messenger of Allah ﷺ saw a boy, some portion of whose head was shaved and some of it was left out. He prohibited them from that and said, "Shave the whole of it or leave the whole of it."

[Muslim].

وعنه قال: رأى رسول الله ﷺ صبيًا قد حلق بعض شعر رأسه وترك بعضه فنهاهم عن ذلك وقال: "احلقوه كله، أو اتركوه كله". رواه أبو داود بإسناد صحيح على شرط البخاري ومسلم.