From Sahih Muslim (Hadith 5553):
Ibn Abbas reported that Allah's Messenger (ﷺ) 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
Reference: Muslim 5553