Musnad Ahmad 258:6

Abu al-Nadr Hashim narrated to us, he said: Ibn 'Alathah informed us, he said: Ibrahim ibn Abi 'Ablah narrated to us, from Wathilah ibn al-Asqa', he said: A group of people from Banu Sulaym came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "O Messenger of Allah, a companion of ours has made [Hellfire] mandatory [upon himself]." The Messenger of Allah, may Allah bless him and grant him peace, said: "Let him free a slave like himself; Allah, the Almighty and Majestic, will release for every limb of [the slave] a limb of his from the Fire."