It was narrated from Zadhan that ‘Ali (رضي الله عنه) said:
I heard the Prophet (ﷺ) say: “Whoever leaves a space the size of a hair on his side when doing ghusl for janabah and does not make water reach it, Allah will punish him with such and such of fire.` `Ali (رضي الله عنه) said: From that time I disliked my head (i.e., hair).