Musnad Ahmad 28:122

Hassan narrated to us, Ibn Lahi'ah narrated to us, Huyayy ibn Abdullah narrated to me, from Abu Abd al-Rahman al-Hubuli, from Abdullah ibn Amr, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Marry the mothers of children (umm al-walad), for I will boast of your numbers on the Day of Resurrection."