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."
سَمِعْتُ أَنَسَ بْنَ مَالِكٍ ؓ يَقُولُ سَبَى النَّبِيُّ ﷺ صَفِيَّةَ فَأَعْتَقَهَا وَتَزَوَّجَهَا فَقَالَ ثَابِتٌ لأَنَسٍ مَا أَصْدَقَهَا قَالَ أَصْدَقَهَا نَفْسَهَا فَأَعْتَقَهَا