Sahih Muslim 213a

Muhammad ibn al-Muthanna and Ibn Bashshar narrated to us; the wording is Ibn al-Muthanna’s. They said: Muhammad ibn Ja‘far narrated to us: Shu‘ba narrated to us, who said: I heard Abu Ishaq say: I heard al-Nu‘man ibn Bashir deliver a sermon and say: I heard the Messenger of Allah (peace and blessings be upon him) say, “The least tormented person among the people of the Fire on the Day of Resurrection will be a man with two embers placed beneath the arches of his feet, from which his brain will boil.”