Al-Harith bin Hisham asked Allah's Apostle O Allah's Messenger How is the Divine Inspiration revealed to you? Allah's Messenger replied, Sometimes it is like the ringing of a bell, this form of Inspiration is the hardest of all and then this state passes off after I have grasped what is inspired. Sometimes the Angel comes in the form of a man and talks to me and I grasp whatever he says. Aisha added: Verily I saw the Prophet being inspired Divinely on a very cold day and noticed the Sweat dropping from his forehead.

Sahih Bukhari vol 1 book 1 hadith 2