Sunan an-Nasa'i Book 48, Hadith 60
"The Messenger of Allah cursed the women who do tattoos and the women who have them done, Al-Mutanammisat, and the women who have their teeth separated for the sake of beauty, those who change (the creation of Allah.)"
A woman came to 'Abdullah bin Mas'ud and said: "I am a woman with little hair, is it alright for me to add extensions to my hair?" He said: "No." She said: "Is it something that you heard from the Messenger of Allah and I find it in the Book of Allah." And he quoted the Hadith.
"Abdullah said: 'The women who have their teeth separated..." and he quoted the Hadith.'"