Musnad Ahmad 1153:1656

Yahya narrated to us, from Abd al-Malik, he said: Ata narrated to us, from Aisha, she said: The Messenger of Allah (peace and blessings of Allah be upon him) would be struck by janabah (major ritual impurity) at night while he intended to fast, so he would perform ghusl (ritual bath) after the dawn had broken, then he would complete his fast.