Musannaf Ibn Abi Shayba 7:489

Yunus ibn Muhammad narrated to us, from Layth ibn Sa'd, from Yazid ibn 'Abd Allah, from 'Umar ibn 'Ali ibn Husayn, from Sa'id ibn Marjana, who said: I heard him narrating from Abu Hurayrah, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "Whoever frees a believing slave, Allah will free for every limb of his, a limb of his own from the Fire, until He frees his private part for his private part."