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. (Using translation from Aḥmad 727)
سَمِعْتُ النَّبِيَّ ﷺ قَالَ مَنْ تَرَكَ مَوْضِعَ شَعْرَةٍ مِنْ جَسَدِهِ مِنْ جَنَابَةٍ لَمْ يُصِبْهَا الْمَاءُ فُعِلَ بِهِ كَذَا وَكَذَا مِنَ النَّارِ قَالَ عَلِيٌّ فَمِنْ ثَمَّ عَادَيْتُ رَأْسِي قَالَ قَالَ عَبْدُ اللهِ فِي حَدِيثِهِ وَكَانَ يَجُزُّ شَعْرَهُ