Musnad Ahmad 962:50

Al-Hasan ibn Siwar narrated to us, Layth ibn Sa’d narrated to us, from Mu’awiyah ibn Salih: that Abu ‘Abd al-Rahman narrated to him, from Abu Umamah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The sun will draw near on the Day of Resurrection to the distance of a mile, and its heat will be increased by such and such. The brains will boil from it just as cooking pots boil. They will sweat in it according to the measure of their sins. Among them are those whose sweat will reach their ankles, and among them are those whose sweat will reach their shins, and among them are those whose sweat will reach their waist, and among them are those whom the sweat will bridle."