Musnad Ahmad 1153:2179

I read to Abd al-Rahman: Malik, from Hisham ibn Urwah, from his father, from Aisha: That al-Harith ibn Hisham asked the Messenger of Allah, peace and blessings of Allah be upon him, and said: "O Messenger of Allah, how does the revelation come to you?" The Messenger of Allah, peace and blessings of Allah be upon him, 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 he said. And sometimes the angel appears to me as a man, and he speaks to me, and I grasp what he says." Aisha said: "And I have seen him when the revelation would descend upon him on an intensely cold day, and then it would cease from him, and his forehead would be dripping with sweat."