Sahih Muslim 154a

Yahya ibn Yahya narrated to us: Hushaim informed us, from Salih ibn Salih al-Hamdani, from al-Sha'bi, who said: I saw a man from Khurasan ask al-Sha'bi: Abu 'Amr, people among us in Khurasan say that a man who frees his enslaved woman and then marries her is like someone who rides his sacrificial camel. Al-Sha'bi said: Abu Burda ibn Abi Musa told me, from his father, that the Messenger of Allah (peace and blessings be upon him) said: Three people will receive their reward twice over: a person from the People of the Book who believed in his prophet, then lived to see the Prophet (peace and blessings be upon him), believed in him, followed him, and affirmed his truth—he will have a double reward; an enslaved person who fulfills his duty to Allah and to his master—he will have a double reward; and a man who has an enslaved woman, provides and cares well for her, teaches her good manners and does so well, then frees her and marries her—he will have a double reward. Al-Sha'bi then told the man from Khurasan: Take this hadith without payment. In the past a man would travel to Medina for even less than this.

Abu Bakr ibn Abi Shayba narrated to us: 'Abda ibn Sulayman narrated to us. H. Ibn Abi 'Umar narrated to us: Sufyan narrated to us. H. 'Ubaydullah ibn Mu'adh narrated to us: My father narrated to us; Shu'ba narrated to us. All of them reported it from Salih ibn Salih through this chain, with a similar wording.