Musnad Ahmad 652:67

Muhammad ibn Ja'far narrated to us, Shu'bah informed us, he said: I heard Abu Ishaq saying: I heard al-Nu'man ibn Bashir, while he was delivering a sermon, 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 embers placed under the arches of his feet, from which his brain will boil."