Musnad Ahmad 32:318

Yahya ibn Sa'id narrated to us, from Yazid, and Rawh, Yazid ibn Abi Salih narrated to us—the meaning being—he said: I heard Anas ibn Malik narrating from the Prophet, may Allah bless him and grant him peace, who said: "Some people from my nation will enter the Fire until, when they have become charcoal, they will be admitted into Paradise. The people of Paradise will say: 'Who are these?' It will be said: 'They are the Jahannamiyyun (those of the Hellfire).'"