Sahih al-Bukhari 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.)
حَدَّثَنَا أَبُو الْوَلِيدِ، حَدَّثَنَا شُعْبَةُ، عَنْ هِشَامِ بْنِ زَيْدٍ، عَنْ أَنَسٍ، قَالَ دَخَلْتُ عَلَى النَّبِيِّ صلى الله عليه وسلم بِأَخٍ لِي يُحَنِّكُهُ، وَهْوَ فِي مِرْبَدٍ لَهُ، فَرَأَيْتُهُ يَسِمُ شَاةً ـ حَسِبْتُهُ قَالَ ـ فِي آذَانِهَا‏.‏