Yahya related to me from Malik that he had heard that Said ibnal-Musayyab and Sulayman ibn Yasar said, "The head wound of the slavein which the bone is bared is a twentieth of his price." (ID)
USC-MSA web (English) reference: Book 43, Hadith 8 (ID)
Arabic reference (ID): Book 43, Hadith 1580 (ID)