Riyad as-Salihin #1541
The Book of Miscellany
It has been narrated that Maimuna bint Al-Harith (May Allah be pleased with her) had set free a slave-girl without the Prophet's permission. When her turn came (the Prophet (ﷺ) used to visit his wives in turns), she made mention of that to him saying, "Did you know I have set slave-girl free?" He said, "Have you, indeed?" She replied, "Yes". He (ﷺ) then remarked, "Had you given her to your maternal uncles, you would have your reward increased"..
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About Riyad as-Salihin
This hadith is #1541 from Riyad as-Salihin, in the Book of The Book of Miscellany. Riyad as-Salihin contains 1,896 hadiths across 20 books.