Musnad Ahmad 1153:1234

Muhammad ibn Bishr narrated to us, Hisham ibn Urwah narrated to us, from his father, from Aisha, that al-Harith ibn Hisham asked the Messenger of Allah, peace and blessings of Allah be upon him: "How does the revelation come to you?" He said: "Sometimes it comes to me like the ringing of a bell, and it is the hardest upon me. Then it ceases from me, and I have grasped what was said. And sometimes the angel comes to me in the form of a man, and I grasp what he says."