Sahih Muslim Book 24, Hadith Number 5287.

Chapter : It is forbidden to beat the animal on the face or cauterise at the face.

Anas reported: We went to Allah’s Messenger (may peace be upon him) as he was in the fold and he was cauterising the animals of the flock and I think (he was cauterising them) on their ears. This hadith has been narrated on the authority of Shu’ba through another chain of transmitters.