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.ʿ ʿAli ؓ said: From that time I disliked my head (i.e., hair).
سَمِعْتُ النَّبِيَّ ﷺ يَقُولُ مَنْ تَرَكَ مَوْضِعَ شَعَرَةٍ مِنْ جَسَدِهِ مِنْ جَنَابَةٍ لَمْ يُصِبْهَا الْمَاءُ فُعِلَ بِهِ كَذَا وَكَذَا مِنَ النَّارِ قَالَ عَلِيٌّ فَمِنْ ثَمَّ عَادَيْتُ رَأْسِي فَمِنْ ثَمَّ عَادَيْتُ رَأْسِي