Sahih Muslim
The Book Pertaining to Clothes and Decoration (Kitab Al-Libas wal-Zinah)
Book Number: 024 / Hadith Number: 5289
Ibn Umar reported that Allah's Messenger (may peace be upon him) forbade Qaza. I said to Nafi: What is Qaza'? He said: This means having a part of a boy's head shaved and leaving a part unshaven.