Mishkat al-Masabih #2697

Jabir told that a slave came and swore allegiance to the Prophet promising to emigrate, but he did not know that he was a slave. When his master came in search of him the Prophet said to him, ‘Sell him to me," and he bought him for two black slaves. Afterwards he never took an oath of allegiance from anyone without asking him whether he was a slave or free. Muslim transmitted it.