Musnad Ahmad 32:954

Waki’ narrated to us, Yazid ibn Abi Salih narrated to us—and he was a tanner, and he was of good appearance, and he had four hadiths—he said: I heard Anas ibn Malik saying: The Messenger of Allah, peace and blessings of Allah be upon him, said: "People will enter Hell until, when they become charcoal, they will be brought out and entered into Paradise. Then the people of Paradise will say: 'These are the Hell-dwellers.'"