Musannaf Ibn Abi Shayba 9:720

Ibn Numayr narrated to us, from ‘Abd al-Malik, from ‘Ata’, regarding a man who bought a slave girl from her parents: She is a virgin. He said: He shall seek to establish the innocence of her womb (istibra’) with two menstrual cycles. And if she does not menstruate, then with forty-five days.