Sahih Bukhari
Hunting, Slaughtering
Volume: 007 / Book Number: 067 / Hadith Number: 450
Narrated Anas:
I brought a brother of mine to the Prophet to do Tahnik for him while the Prophet was in a sheep fold of his, and I saw him branding a sheep. (The sub-narrator said: I think Anas said, branding it on the ear.)