Sayyidah Aishah (may Allah be pleased with her) narrates from Sayyiduna al-Harith bin Hisham (may Allah be pleased with him) that he asked the Prophet (peace and blessings be upon him): How does the revelation come down to you? The Messenger of Allah (peace and blessings be upon him) said: "It comes like the ringing of a bell; then that state leaves me, and I have retained all that was said." He (peace and blessings be upon him) said: "And this is the hardest state on me. And sometimes the angel comes to me, takes on a (human) form, and speaks to me what he says."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3330