I heard the Messenger of Allah ﷺ say, "Whoever frees a believing slave, Allah will free from the Fire one of his limbs for every limb of the freed slave, until He frees his private parts in return for the private parts."
حدثنا قتيبة حدثنا الليث عن ابن الهاد عن عمر بن علي بن الحسين بن علي بن أبي طالب عن سعيد ابن مرجانة
سمعت رسول الله ﷺ يقول من أعتق رقبة مؤمنة أعتق الله منه بكل عضو منه عضوا من النار حتى يعتق فرجه بفرجه
Agreed upon (muttafaq alayh). Muslim narrated it in the Book of Manumission, chapter on the virtue of manumission, hadith no. 23/1509, from Qutayba, and Bukhari, hadith no. 6715, from the narration of Saeed bin Marjana, with the same wording.This subject is also reported from Ayesha [Tahawi in Mushkil Athar: 1/310], Amr bin Abasa (Abu Najeeh) [Abu Dawud, no. 3966], Ibn Abbas [Tabarani in Al-Kabir: 10/331, 332, no. 10640, 10641, and its chain is daif], Waila bin Asqa [Abu Dawud, no. 3964], Abu Umama [to come: 1547], Uqba bin Amir [Ahmad: 4/147, 150], and Kaab bin Murra [Abu Dawud, no. 3967].