Musannaf Ibn Abi Shayba 9:756

Mu'tamir narrated to us, from Kathir ibn Yasar, from Ibn Sirin, who said: If a man buys a slave girl and she has not reached menstruation, he shall wait for her to be clear (istibra') for three months. If he has had intercourse with her and then wishes to sell her, he must also wait for her to be clear (istibra') for three months.