Anas bin Malik reported,

We came upon the Messenger of Allah ﷺ in an animal pen while he was marking some sheep. The narrator added, "I believe he said it was on their ears."

Muslim 5557

Sahih

و حدثنيه يحيى بن حبيب حدثنا خالد بن الحارث ح و حدثنا محمد بن بشار حدثنا محمد ويحيى وعبد الرحمن كلهم عن شعبة بهذا الإسناد مثله

مسلم 5557صحيح
Contents
Reference
Commentary
Ayat
Collection
Sahih Muslim
Book
Clothing and adornment
Chapter
Permissibility of branding non-human animals on other than the face and its recommendation for zakat and jizya livestock
Numbering
Darussalam 5557
Muslim 5557