Sahih al-Bukhari, Volume 8, Book 76, Hadith 566 (USC-MSA) ⓘ

Narrated An-Nu`man: I heard the Prophet saying, "The person who will have the least punishment from amongst the Hell Fire people on the Day of Resurrection, will be a man under whose arch of the feet a smoldering ember will be placed so that his brain will boil because of it."

حَدَّثَنِي مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا غُنْدَرٌ، حَدَّثَنَا شُعْبَةُ، قَالَ سَمِعْتُ أَبَا إِسْحَاقَ، قَالَ سَمِعْتُ النُّعْمَانَ، سَمِعْتُ النَّبِيَّ صلى الله عليه وسلم يَقُولُ ‏ "‏إِنَّ أَهْوَنَ أَهْلِ النَّارِ عَذَابًا يَوْمَ الْقِيَامَةِ لَرَجُلٌ تُوضَعُ فِي أَخْمَصِ قَدَمَيْهِ جَمْرَةٌ يَغْلِي مِنْهَا دِمَاغُهُ‏" ‏.‏