Maliks Muwatta Book 038, Hadith Number 015.

Section : The Excellence of Freeing Slaves, Freeing Adulteresses and Illegitimate Children.

Malik related to me from Hisham ibn Urwa from his father from A'isha, the wife of the Prophet, may Allah bless him and grant him peace, that the Messenger of Allah, may Allah bless him and grant him peace, was asked what was the most excellent kind of slave to free. The Messenger of Allah, May Allah bless him and grant him peace, answered, "The most expensive and the most valuable to his master."