Jabir (Allah be pleased with him) reported: There came a slave and pledg- ed allegiance to Allah's Apostle ﷺ on migration;
he (the Holy Prophet) did not know that he was a slave. Then there came his master and demanded him back, whereupon Allah's Apostle ﷺ said: Sell him to me. And he bought him for two black slaves, and he did not afterwards take allegiance from anyone until he had asked him whether he was a slave (or a free man)

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى التَّمِيمِيُّ، وَابْنُ، رُمْحٍ قَالاَ أَخْبَرَنَا اللَّيْثُ، ح وَحَدَّثَنِيهِ قُتَيْبَةُ، بْنُ سَعِيدٍ حَدَّثَنَا لَيْثٌ، عَنْ أَبِي الزُّبَيْرِ، عَنْ جَابِرٍ، قَالَ جَاءَ عَبْدٌ فَبَايَعَ النَّبِيَّ ﷺ عَلَى الْهِجْرَةِ وَلَمْ يَشْعُرْ أَنَّهُ عَبْدٌ فَجَاءَ سَيِّدُهُ يُرِيدُهُ فَقَالَ لَهُ النَّبِيُّ ﷺ " بِعْنِيهِ " . فَاشْتَرَاهُ بِعَبْدَيْنِ أَسْوَدَيْنِ ثُمَّ لَمْ يُبَايِعْ أَحَدًا بَعْدُ حَتَّى يَسْأَلَهُ " أَعَبْدٌ هُوَ " .