Musnad Ahmad 32:1734

Rawh narrated to us, Yazid ibn Abi Salih narrated to us, he said: I heard Anas ibn Malik narrating from the Prophet, peace and blessings of Allah be upon him: "People will enter the Fire until, when they have become charcoal, they will be admitted into Paradise. Then the people of Paradise will say: 'Who are these?' It will be said: 'These are the Jahannamiyyun (the people of Hell).'"