Musnad Ahmad 31:605

Yahya ibn Ishaq and Aswad ibn Amir narrated to us, they said: Sharik informed us, from Abu Ishaq and Qays ibn Wahb, from Abu al-Wadak, from Abu Sa’id al-Khudri, that the Messenger of Allah (may Allah bless him and grant him peace) said regarding the captives of Awtas: "A pregnant woman shall not be had intercourse with—Aswad said: until she delivers—and a non-pregnant woman shall not be had intercourse with until she has a menstrual cycle." Yahya said: "Or until she is cleared by a menstrual cycle."