كتاب اللباس والزينة
37  
The Book of Clothes and Adornment
(30)
Chapter: The Permissibility Of Branding Animals Anywhere But On The Face, And This Is Recommended In The Case Of Animals Given As Zakat Ot Jizyah
(30)
بَاب جَوَازِ وَسْمِ الْحَيَوَانِ غَيْرِ الْآدَمِيِّ فِي غَيْرِ الْوَجْهِ وَنَدْبِهِ فِي نَعَمْ الزَّكَاةِ وَالْجِزْيَةِ
Sahih Muslim 2119 b

Anas reported that when his mother gave birth to a child they brought that child to Allah's Messenger (ﷺ) so that he might chew some dates and touch his palate with them. and Allah's Apostle (ﷺ) was at that time in the fold busy in cauterising the animals Shu'ba said:

So far as I know (he was cauterising) their ears.
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ هِشَامِ بْنِ زَيْدٍ، قَالَ سَمِعْتُ أَنَسًا، يُحَدِّثُ أَنَّ أُمَّهُ، حِينَ وَلَدَتِ انْطَلَقُوا بِالصَّبِيِّ إِلَى النَّبِيِّ صلى الله عليه وسلم يُحَنِّكُهُ قَالَ فَإِذَا النَّبِيُّ صلى الله عليه وسلم فِي مِرْبَدٍ يَسِمُ غَنَمًا ‏.‏ قَالَ شُعْبَةُ وَأَكْثَرُ عِلْمِي أَنَّهُ قَالَ فِي آذَانِهَا ‏.‏
Reference : Sahih Muslim 2119b
In-book reference : Book 37, Hadith 166
USC-MSA web (English) reference : Book 24, Hadith 5286
  (deprecated numbering scheme)