(6) باب فِي الرَّجُلِ يَعْتِقُ أَمَتَهُ ثُمَّ يَتَزَوَّجُهَا
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."
Reference: Sunan Abi Dawud 2053
In-book reference: Book 12, Hadith 8
English translation: Book 11, Hadith 2048