Sahih al-Bukhari 2594
Arabic
وَقَالَ بَكْرٌ عَنْ عَمْرٍو، عَنْ بُكَيْرٍ، عَنْ كُرَيْبٍ، مَوْلَى ابْنِ عَبَّاسٍ إِنَّ مَيْمُونَةَ زَوْجَ النَّبِيِّ صلى الله عليه وسلم أَعْتَقَتْ وَلِيدَةً لَهَا فَقَالَ لَهَا " وَلَوْ وَصَلْتِ بَعْضَ أَخْوَالِكِ كَانَ أَعْظَمَ لأَجْرِكِ ".
English translation
Narrated Maimuna, the wife of the Prophet (ﷺ) that she manumitted her slave-girl and the Prophet (ﷺ) said to her, "You would have got more reward if you had given the slave-girl to one of your maternal uncles
Reference: Sahih al-Bukhari 2594, Book of Gifts. English translation by Dr. Muhammad Muhsin Khan.