Al-Harith bin Hisham (may Allah be pleased with him) asked him (peace and blessings be upon him): How does the revelation come to you? The Prophet (peace and blessings be upon him) said: "Sometimes the revelation comes like the ringing of a bell, and this is the hardest on me; then it leaves me, and I have retained what the angel said. And sometimes the angel comes in the form of a man; he teaches me and I retain what he says."
Sayyidah Aishah (may Allah be pleased with her) said: I saw the revelation descending upon him (peace and blessings be upon him) on a severely cold day; then it left him (peace and blessings be upon him), and his (peace and blessings be upon him) forehead was dripping with sweat.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13342