Abu Dawud reported the Apostle of Allaah ﷺ as saying “Anyone who sets his slave girl free and then marries her, will have a double reward.” (Using translation from Abū Dāʾūd 2053)
قَالَ رَسُولُ اللهِ ﷺ مَنْ كَانَتْ لَهُ جَارِيَةٌ فَأَعْتَقَهَا وَتَزَوَّجَهَا كَانَ لَهُ أَجْرَانِ