Musnad Ahmad 157:1

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu al-Fayd, he said: I heard 'Abdullah ibn Murrah narrating from Abu Sa'id al-Zurqi, that a man from Ashja' asked the Prophet, peace and blessings of Allah be upon him, about *al-'azl* (coitus interruptus). He said: "My wife is breastfeeding." The Prophet, peace and blessings of Allah be upon him, said: "Indeed, whatever is decreed in the womb will come to pass."