Musnad Ahmad 1153:183

Abu Mu'awiyah narrated to us, Hisham ibn 'Urwah narrated to us, from his father, from 'Aishah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, used to have young children brought to him, and he would supplicate for them. And indeed, a young boy was brought to him, and he urinated on him. So the Messenger of Allah, peace and blessings of Allah be upon him, said: "Pour water over it, a pouring."