From Sahih Bukhari (Hadith 5542):
Narrated Anas:I brought a brother of mine to the Prophet (ﷺ) to do Tahnik for him while the Prophet (ﷺ) 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)
Reference: Bukhari 5542