Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates a marfu' hadith: "Concerning the captives of Awtas, the ruling is that a pregnant woman is not to be approached for intercourse until she gives birth, and for one who is not pregnant, one is to wait for one menstrual period."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10791