Volume 3 · Book 46 · Hadith 727
Narrated Abu Musa: The Prophet said, "The Mamluk (slave) who worships his Lord in a perfect manner, and is dutiful, sincere and obedient to his Saiyid (master), will get a double reward."
— Sahih Bukhari, Volume 3, Book 46: Manumission of Slaves, Hadith #727