Sahih Muslim Book - 1, Hadith - 517

و حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ حَدَّثَنَا أَبُو أُسَامَةَ عَنْ الْأَعْمَشِ عَنْ أَبِي إِسْحَاقَ عَنْ النُّعْمَانِ بْنِ بَشِيرٍ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِنَّ أَهْوَنَ أَهْلِ النَّارِ عَذَابًا مَنْ لَهُ نَعْلَانِ وَشِرَاكَانِ مِنْ نَارٍ يَغْلِي مِنْهُمَا دِمَاغُهُ كَمَا يَغْلِي الْمِرْجَلُ مَا يَرَى أَنَّ أَحَدًا أَشَدُّ مِنْهُ عَذَابًا وَإِنَّهُ لَأَهْوَنُهُمْ عَذَابًا.

It was narrated that An-Nu'man bin Bashir said: "The Messenger of Allah (s.a.w) said: 'The least severely punished of the people of the Fire will be a man who has sandals and sandal-straps of fire, because of which his brain will boil as a cooking pot boils. He will think that no one else is being punished as severely as he, but he will be the least severely punished of them.'"