صحيح مسلم

It is narrated on the authority of Sha'bi that one among the citizens of Khurasan asked him: 0 Abu! some of the people amongst us who belong to Khurasan say that a person who freed his bondswoman and then married her is like one who rode over a sacrificial animal. Sha'bi said: Abu Burda b. Abi Musa narrated it to me on the authority of his father that verily the Messenger of Allah (ﷺ) said: There are three (classes of persons) who would be given a double reward. One who is amongst the People of the Book and believed in his apostle and (lived) to see the time of Apostle Muhammad (ﷺ) and affirmed his faith in him and followed him and attested his truth, for him is the double reward; and the slave of the master who discharges all those obligations that he owes to Allah and discharges his duties that he owes to his master, for him there is a double reward. And a man who had a bondswoman and fed her and fed her well, then taught her good manners, and did that well and later on granted her freedom and married her, for him is the double reward. Then Sha'bi said: Accept this hadith without (giving) anything. Formerly a man was (obliged) to travel to Medina even for a smaller hadith than this.

This hadith has been narrated by another chain of transmitters like Abu Bakr b. Abi Shaiba, 'Abda b. Sulaiman Ibn Abi 'Umar Sufyan, 'Ubaidullah b. Mu'adh, Shu'ba; all of them heard it from Salih b. Salih.