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 ؓ said: From that time I hated my hair, as you can see.
سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ مَنْ تَرَكَ مَوْضِعَ شَعَرَةٍ مِنْ جَنَابَةٍ لَمْ يُصِبْهَا الْمَاءُ فُعِلَ بِهِ كَذَا وَكَذَا مِنَ النَّارِ قَالَ عَلِيٌّ فَمِنْ ثَمَّ عَادَيْتُ شَعْرِي كَمَا تَرَوْنَ