Sunan an-Nasa'i Book 48, Hadith 55
The Messenger of Allah cursed the woman who affixes hair extensions and the woman who has that done.
"I saw Mu'awiyah bin Abi Sufyan on the Minbar, holding a ball of hair such as women use. He said: "What is wrong with Muslim women who put such things (on their heads)? I heard the Messenger of Allah say: "Any woman who adds hair to her head that is not hers, it is something false, that she is adding to her head."
"The Messenger of Allah cursed the woman who affixes hair extensions and the woman who has that done, the woman who does tattoos and the woman who has that done."