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)   

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ «مَنْ تَرَكَ مَوْضِعَ شَعْرَةٍ مِنْ جَنَابَةٍ لَمْ يُصِبْهَا الْمَاءُ فُعِلَ بِهَا كَذَا وَكَذَا مِنَ النَّارِ» قَالَ عَلِيٌّ فَمِنْ ثَمَّ عَادَيْتُ رَأْسِي وَكَانَ يَجُزُّ شَعْرَهُ