Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) says: On the day of Awtas we obtained female captives, and the Messenger of Allah (peace and blessings be upon him) said: "No pregnant woman is to be approached for intercourse until she gives birth, nor a non-pregnant one until she has had one menstrual period."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18297