Sahih al-Bukhari 6561
Sahih al-Bukhari · Book 81: To make the Heart Tender (Ar-Riqaq) · The description of Paradise and the Fire
Narrated An-Nu`man:
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".
I heard the Prophet (ﷺ) 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."