Musnad Ahmad 652:44

Yahya ibn Sa'id narrated to us, from Shu'bah, who said: Abu Ishaq narrated to me, saying: I heard al-Nu'man ibn Bashir delivering a sermon, and he was saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Indeed, the person among the people of the Fire with the lightest punishment on the Day of Resurrection is a man who will have two sandals of fire placed under the arches of his feet, from which his brain will boil."