I heard the Prophet ﷺ say, "The person receiving the lightest punishment among the people of Hell on the Day of Resurrection will be a man with a burning ember placed under the arch of his foot, causing his brain to boil."
سمعت النبي ﷺ يقول: إن أهون أهل النار عذابا يوم القيامة لرجل توضع في أخمص قدميه جمرة يغلي منها دماغه.
Narrated by Bukhari in the Book of Riqaq (6561) and Muslim in the Book of Iman (213), both from Muhammad bin Bashshar, who said, Muhammad bin Jafar told us, Shubah told us, he said, I heard Abu Ishaq say, I heard Numan bin Bashir mention it. The wording given is Bukhari's.