Muwatta Malik 1477

حَدَّثَنِي مَالِكٌ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، زَوْجِ النَّبِيِّ صلى الله عليه وسلم أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم سُئِلَ عَنِ الرِّقَابِ أَيُّهَا أَفْضَلُ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ أَغْلاَهَا ثَمَنًا وَأَنْفَسُهَا عِنْدَ أَهْلِهَا ‏"‏ ‏.‏

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

Reference

Muwatta Malik 1477

In-book

Book 38, Hadith 14