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].
App reference: Book 18, Hadith 129
Arabic/English book reference: Book 18, Hadith 1639