Musnad Ahmad 30:2645

Waki’ narrated to us, he said: ‘Abdullah bin Sa’id—meaning Ibn Abi Hind—narrated to me, from Sa’id bin Marjanah, that he narrated to ‘Ali bin Husayn, from Abu Hurayrah, that the Messenger of Allah (ﷺ) said: "Whoever frees a slave, for every limb of his [the slave] that is freed, a limb of his [the master] will be freed from the Fire," until he mentioned the private part. He [the narrator] said: So ‘Ali bin Husayn called for a servant of his and freed him.