Language:
كتاب الرقاق
81  
To make the Heart Tender (Ar-Riqaq)
(51)
Chapter: The description of Paradise and the Fire
(51)
باب صِفَةِ الْجَنَّةِ وَالنَّارِ
Sahih al-Bukhari 6562

Narrated An-Nu`man bin Bashir:

I heard the Prophet (ﷺ) saying, "The least punished person of the (Hell) Fire people on the Day of Resurrection will be a man under whose arch of the feet two smoldering embers will be placed, because of which his brain will boil just like Al-Mirjal (copper vessel) or a Qum-qum (narrow-necked vessel) is boiling with water."

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ رَجَاءٍ، حَدَّثَنَا إِسْرَائِيلُ، عَنْ أَبِي إِسْحَاقَ، عَنِ النُّعْمَانِ بْنِ بَشِيرٍ، قَالَ سَمِعْتُ النَّبِيَّ صلى الله عليه وسلم يَقُولُ ‏ "‏ إِنَّ أَهْوَنَ أَهْلِ النَّارِ عَذَابًا يَوْمَ الْقِيَامَةِ رَجُلٌ عَلَى أَخْمَصِ قَدَمَيْهِ جَمْرَتَانِ يَغْلِي مِنْهُمَا دِمَاغُهُ، كَمَا يَغْلِي الْمِرْجَلُ وَالْقُمْقُمُ‏"‏‏.‏
Reference : Sahih al-Bukhari 6562
In-book reference : Book 81, Hadith 151
USC-MSA web (English) reference : Vol. 8, Book 76, Hadith 567
  (deprecated numbering scheme)