Jabir (Allah be pleased with him) reported: There came a slave and pledg- ed allegiance to Allah's Messenger (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 Messenger (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)
Source: sunnah.com
Other translations
Arabic
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 صلى الله عليه وسلم عَلَى الْهِجْرَةِ وَلَمْ يَشْعُرْ أَنَّهُ عَبْدٌ فَجَاءَ سَيِّدُهُ يُرِيدُهُ فَقَالَ لَهُ النَّبِيُّ صلى الله عليه وسلم " بِعْنِيهِ " . فَاشْتَرَاهُ بِعَبْدَيْنِ أَسْوَدَيْنِ ثُمَّ لَمْ يُبَايِعْ أَحَدًا بَعْدُ حَتَّى يَسْأَلَهُ " أَعَبْدٌ هُوَ " .
sunnah.com