Musnad Ahmad 1153:1184

Yahya ibn Adam narrated to us, Ibn al-Mubarak narrated to us, from Ma'mar and Yunus, from al-Zuhri, from 'Urwah, from 'Aishah, she said: "The first thing with which the Messenger of Allah, peace and blessings of Allah be upon him, began regarding the revelation was the truthful vision, or she said: the righteous vision—Ibn al-Mubarak was in doubt." She said: "And he would not see a vision except that it came like the breaking of the dawn."