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

عَنْ عَائِشَةَ زَوْجِ النَّبِيِّ ﷺ أَنَّ رَسُولَ اللَّهِ ﷺ سُئِلَ عَنِ الرِّقَابِ أَيُّهَا أَفْضَلُ فَقَالَ رَسُولُ اللَّهِ ﷺ أَغْلاَهَا ثَمَنًا وَأَنْفَسُهَا عِنْدَ أَهْلِهَا  

[Machine] He informed him that he said: "O Messenger of Allah, which deed is best?" He said: "Belief in Allah and Jihad in His cause." He said: "Which slaves are best?" He said: "The most expensive and valuable among them to their owners."  

أَخْبَرَهُ أَنَّهُ قَالَ يَا رَسُولَ اللهِ أَيُّ الْعَمَلِ أَفْضَلُ قَالَ «إِيمَانٌ بِاللهِ وَجِهَادٌ فِي سَبِيلِهِ» قَالَ فَأَيُّ الرِّقَابِ أَفْضَلُ؟ قَالَ «أَغْلَاهَا ثَمَنًا وَأَنْفَسُهَا عِنْدَ أَهْلِهَا»  

“I said: 'O Messenger of Allah ﷺ, which slave is best?' He said: ‘The one who is most precious to his master and most valuable in price.' ” (Using translation from Ibn Mājah 2523)   

"أفْضَلُ الرِّقابِ: أغْلاهَا ثمنًا، وأنْفَسُها عندَ أهْلِها" .