I heard the Prophet ﷺ say: ʿWhoever leaves a space the size of a hair when doing ghusl for janabah and does not make water reach it, Allah will punish him with such and such in the Fire.” ‘Ali ؓ said: From that time I disliked my hair.
سَمِعْتُ النَّبِيَّ ﷺ يَقُولُ مَنْ تَرَكَ مَوْضِعَ شَعَرَةٍ مِنْ جَنَابَةٍ لَمْ يُصِبْهَا مَاءٌ فَعَلَ اللهُ تَعَالَى بِهِ كَذَا وَكَذَا مِنَ النَّارِ قَالَ عَلِيٌّ فَمِنْ ثَمَّ عَادَيْتُ شَعْرِي