It is narrated from Sayyiduna Wathilah bin al-Asqa' (may Allah be pleased with him) that a man came to the Prophet (peace and blessings be upon him) and said: "O Messenger of Allah, what do you say about a man who has made it obligatory?" meaning, he has become deserving of the Fire. He (peace and blessings be upon him) said: "Let him free a slave, for every limb of it (the slave) will become a ransom freeing a limb of his from the Fire."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18442