Sahih Muslim | Book 24 | Hadith Number 5284

Narrated by Ibn Abbas
Ibn Abbas reported that Allah's Messenger (may peace be upon him) saw an ass which had been cauterised on the face. He disapproved of it saying: By Allah, I do not cauterise (the animal) but on a part at a distance from the face, and commanded (for the cauterisation) of his ass and it was cauterised on the buttocks and he was the first to cauterise on the buttocks.