Malik related to me from Hisham ibn Urwa from his father fromA'isha, the wife of the Prophet, may Allah bless him and grant himpeace, that the Messenger of Allah, may Allah bless him and grant himpeace, 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."
USC-MSA web (English) reference: Book 38, Hadith 15
Arabic reference: Book 38, Hadith 1480