Musannaf Ibn Abi Shayba 9:1560

Abu al-Ahwas narrated to us, from Abu Ishaq, from Silah, Qutham, and Najiyah ibn Ka'b, they said: "Any man who buys a pregnant slave girl, he shall not have intercourse with her until she delivers her burden. And any man who buys a slave girl, he shall not approach her until she has menstruated."