Musnad Ahmad 448:10

Hassan ibn Musa narrated to us, he said: Ibn Lahi'ah narrated to us, from al-Harith ibn Yazid, he said: Hanash narrated to me, he said: We were with Ruwayfi' ibn Thabit on the expedition of Jarba, and he divided it [the spoils] among us. And Ruwayfi' said to us: Whoever obtains any of these captives, let him not have intercourse with her until she has menstruated, for I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: "It is not lawful for a man to water his seed into the child of another."