Sahih Muslim
The Book Pertaining to Clothes and Decoration (Kitab Al-Libas wal-Zinah)
Book Number: 024 / Hadith Number: 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).