Sunan an-Nasa'i 4184
It was narrated that jabir said:
"A slave came and pledged to the Prophet to emigrate, and the Prophet did not realize that he was a slave. Then his master came looking for him. The Prophet said: 'Sell him to me,' and he bought him for two black slaves. Then he did not accept the pledge from anyone until he asked: 'Is he a slave?"'
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ أَبِي الزُّبَيْرِ، عَنْ جَابِرٍ، قَالَ جَاءَ عَبْدٌ فَبَايَعَ النَّبِيَّ صلى الله عليه وسلم عَلَى الْهِجْرَةِ وَلاَ يَشْعُرُ النَّبِيُّ صلى الله عليه وسلم أَنَّهُ عَبْدٌ فَجَاءَ سَيِّدُهُ يُرِيدُهُ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏
"‏ بِعْنِيهِ ‏"‏ ‏.‏ فَاشْتَرَاهُ بِعَبْدَيْنِ أَسْوَدَيْنِ ثُمَّ لَمْ يُبَايِعْ أَحَدًا حَتَّى يَسْأَلَهُ أَعَبْدٌ هُوَ