Sunan An-Nasa'i

Narrated Abu Hurairah:It was narrated from Abu Hurairah that the Messenger of Allah said: ""There are three who are promised the help of Allah: The Mukatab who wants to buy his freedom, the one who gets married seeking to keep himself chaste, and the Mujahid who fights in the cause of Allah."" *Mukatab: the slave who has made a contract of manumission