Musannaf Ibn Abi Shayba 9:138

Yahya ibn Abd al-Malik ibn Abi Ghaniyah narrated to us, from his father, from al-Hakam, who said: If a man gives permission to his slave regarding a slave-girl to purchase her, that he may have intercourse with her, then it is a marriage from the master. And he shall not have intercourse with more than two.