The Book Pertaining to Clothes and Decoration (Kitab Al-Libas wa'l-Zinah)
Muslim :: Book 24 : Hadith 5283
Jabir reported that there happened to pass before Allah's Apostle (may peace be upon him) an ass the face of which had been cauterised, whereupon he said: Allah has cursed one who has cauterised it (on the face).