I asked, "Messenger of Allah, which slave is best to free?"He replied, "The one most prized by their owners and the highest in price."
حدثنا أحمد بن سنان حدثنا أبو معاوية حدثنا هشام بن عروة عن أبيه عن أبي مراوح
قلت يا رسول الله أي الرقاب أفضل قال أنفسها عند أهلها وأغلاها ثمنا
Bukhari narrated it in Sahih Bukhari, Book of Manumission, chapter on which slaves are most excellent to free, hadith no. 2518, and Muslim, Book of Faith, chapter on the statement that faith in Allah Most High is the most excellent of deeds, hadith no. 84, from the hadith of Hisham, at greater length.