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
“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.' ”
قُلْتُ يَا رَسُولَ اللَّهِ أَىُّ الرِّقَابِ أَفْضَلُ قَالَ أَنْفَسُهَا عِنْدَ أَهْلِهَا وَأَغْلاَهَا ثَمَنًا
“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" .