A woman came to the Messenger of Allah [SAW] and said: "O Messenger of Allah [SAW], a daughter of mine is going to get married. She got sick and her hair fell out. Is there any sin on me if I give her hair extensions?" He said: "Allah has cursed the woman who affixes hair extensions and the one who has that done." (Using translation from Nasāʾī 5250)  

أَنَّ امْرَأَةً جَاءَتْ رَسُولَ اللهِ ﷺ فَقَالَتْ يَا رَسُولَ اللهِ «إِنَّ ابْنَةً لِي عُرْسٌ وَإِنَّهَا اشْتَكَتْ فَتَمَزَّقَ شَعْرُهَا فَهَلْ عَلَيَّ جُنَاحٌ إِنْ وَصَلْتُ لَهَا فِيهِ؟» فَقَالَ «لَعَنَ اللهُ الْوَاصِلَةَ وَالْمُسْتَوْصِلَةَ»  

The Messenger of Allah [SAW] cursed the woman who affixes hair extensions and the woman who has that done. (Using translation from Nasāʾī 5094)  

أَنَّ رَسُولَ اللهِ ﷺ «لَعَنَ الْوَاصِلَةَ وَالْمُسْتَوْصِلَةَ»