Musnad Ahmad 30:1743

Ibrahim narrated to us, Ibn al-Mubarak narrated to us, from Sa'id ibn Yazid, from Abu al-Samh, from Ibn Hujayrah, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Indeed, the boiling water will be poured over their heads, and it will penetrate the skull until it reaches his insides. It will strip away what is in his insides until it passes out through his feet."