The Book of Marriage
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا خَالِدُ بْنُ عَبْدِ اللَّهِ، عَنْ مُطَرِّفٍ، عَنْ عَامِرٍ، عَنْ أَبِي بُرْدَةَ، عَنْ أَبِي مُوسَى، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم فِي الَّذِي يُعْتِقُ جَارِيَتَهُ ثُمَّ يَتَزَوَّجُهَا " لَهُ أَجْرَانِ " .Abu Musa reported that Allah's Messenger (ﷺ) said about one who emancipated a slave woman, and then married her, that for him there are two rewards
From Sahih Muslim, whose reports the scholars accept as authentic.