Musnad Abu Ya'la al-Mawsili 249:2

Al-Abbas ibn al-Walid narrated to us, Ibn al-Mubarak narrated to us, Ibrahim ibn Abi Abla narrated to us, al-Ghurayf ibn Ayyash ibn Fayruz al-Daylami narrated to us, from Wathilah ibn al-Asqa’, who said: Some people from Banu Sulaym came to the Prophet, may Allah bless him and grant him peace, and said: "A companion of ours has made [the Fire] obligatory [upon himself]." He said: "Then let him free a slave, so that Allah may release for every limb of it a limb of his from the Fire."