Musnad Ahmad 448:3

Yahya ibn Ishaq narrated to us, he said: Ibn Lahi'ah informed us; and Qutaybah ibn Sa'id said: Ibn Lahi'ah narrated to 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) said: "It is not lawful for anyone—and Qutaybah said: for a man—to water his water [semen] into the child of another, nor to have intercourse with a slave woman until she has menstruated or her pregnancy has become apparent."