Sahih al-Bukhari 4201
Narrated `Abdul `Aziz bin Suhaib:
Anas bin Malik said, "The Prophet (ﷺ) took Safiya as a captive. He manumitted her and married her." 
Thabit asked Anas, "What did he give her as Mahr (i.e. marriage gift)?" Anas replied. "Her Mahr was 
herself, for he manumitted her."
حَدَّثَنَا آدَمُ، حَدَّثَنَا شُعْبَةُ، عَنْ عَبْدِ الْعَزِيزِ بْنِ صُهَيْبٍ، قَالَ سَمِعْتُ أَنَسَ بْنَ مَالِكٍ ـ رضى الله عنه ـ يَقُولُ سَبَى النَّبِيُّ صلى الله عليه وسلم صَفِيَّةَ، فَأَعْتَقَهَا وَتَزَوَّجَهَا‏.‏ فَقَالَ ثَابِتٌ لأَنَسٍ مَا أَصْدَقَهَا قَالَ أَصْدَقَهَا نَفْسَهَا فَأَعْتَقَهَا‏.‏