It is narrated from Sayyidah Aishah (may Allah be pleased with her) that Sayyiduna al-Harith bin Hisham (may Allah be pleased with him) asked the Messenger of Allah (peace and blessings be upon him): How does the revelation come to you? The Prophet (peace and blessings be upon him) said: "Sometimes it comes to me like the ringing of a bell, and this is the hardest on me; then that state leaves me, and I have retained all that was said. And sometimes the angel appears before me in the form of a man and speaks to me, and I retain all that he says." Sayyidah Aishah (may Allah be pleased with her) said: I saw him (peace and blessings be upon him) when the revelation came down upon him on a bitterly cold day, and when the state left him, his forehead (peace and blessings be upon him) would be dripping with sweat.
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3332