Musnad Ahmad 448:4

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah informed us, from al-Harith ibn Yazid, from Hanash al-San'ani, from Ruwayfi' ibn Thabit, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade that a slave woman be had intercourse with until she has menstruated. And [he forbade it] regarding those who are pregnant until they deliver what is in their wombs."