Musnad Ahmad 1153:1285

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Hisham ibn 'Urwah, from his father, from 'Aishah, who said: A man asked the Prophet, may Allah bless him and grant him peace, and said: "How does the revelation come to you, O Prophet of Allah?" He said: "It comes to me sometimes like the ringing of a bell, and that is the hardest for me. Then it ceases, and I have grasped what was said. And sometimes the angel comes to me in the form of a man, and he speaks to me, and I grasp what he says."