Sayyidah Aishah (may Allah be pleased with her) said: Sayyiduna al-Harith bin Hisham (may Allah be pleased with him) asked the Messenger of Allah (peace and blessings be upon him) how the revelation came to him. He (peace and blessings be upon him) said: "Sometimes it comes to me like the ringing of a bell; then that state leaves me, and I have retained it, and this is the hardest on me. And sometimes the angel comes to me in the form of a man, and I grasp what he says."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3333