Yahya related to me from Malik from Yahya ibn Said from Said ibnal-Musayyab that Umar ibn al-Khattab said, "Anyone who has braided hishair, or plaited it or matted it must shave his head." (ES)
USC-MSA web (English) reference: Book 20, Hadith 201 (ES)
Arabic reference (ES): Book 20, Hadith 900 (ES)