Narrated Abu Musa Al-Ash`ari: The Prophet (peace be upon him) said,
“He who has a slave-girl and teaches her good manners and improves her education and then manumits and marries her, will get a double reward; and any slave who observes Allah’s right and his master’s right will get a double reward.”
Sahih al-Bukhari, Vol. 3, Book 49, Hadith 31