Zakariyya bin Abi Za'idah says: Amir was asked about a man who buys a slave-girl: may he have intercourse with her before establishing that her womb is clear (istibra')? He said: The Muslims obtained women captives on the day of Awtas, and the Messenger of Allah (peace and blessings be upon him) said: "No one is to go near a pregnant woman until she gives birth, and for one who is not pregnant, one is to wait for one menstrual period."
Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: When a slave-girl is bought, one is to wait for one menstrual period to establish that her womb is clear.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10792