It was narrated that ‘Ali (رضي الله عنه) said: I heard the Messenger of Allah (ﷺ) say: Whoever leaves a space the size of a hair [when doing ghusl for janabah] and does not pour water on it, such and such will be done to him in the Fire. ‘Ali (رضي الله عنه) said: From that time I hated my hair, as you can see.