Reference: I’laam al-Mu’aasireen bi-Fataawa Ibn ‘Uthaymeen – Pages189-190
Question: The transplanting of hair of those who have suffered from alopecia, is done by taking hair from the back of the head and transplanting it in the affected area. So is this permissible?
Response: Yes, it is permissible; Because this is done from the aspect of restoring something to the state in which Allaah [first] created it, as well as from the aspect of removing a defect. This is not [to be considered] from the aspect of beautification, or exceding that which Allaah ‘Azza wa Jall has created, so it is [therefore] not [to be considered] from the aspect of changing the creation of Allaah. Rather, it is restoring that which is missing and [thereby] removing a defect.
And it is not hidden [to you] that which occurs in the story of the three men, one of whom was bald, who expressed his desire that Allaah ‘Azza wa Jall restores his hair; So the angel wiped his head and Allaah restored his hair and blessed him with [a] good [head of] hair.