Sahih Bukhari Volume 001, Book 001, Hadith Number 002.

Narrated By 'Aisha: (The mother of the faithful believers) Al-Harith bin Hisham asked Allah's Apostle صلى الله عليه وآله وسلم‎ "O Allah's Apostle! صلى الله عليه وآله وسلم‎ How is the Divine Inspiration revealed to you?" Allah's Apostle صلى الله عليه وآله وسلم‎ replied, "Sometimes it is (revealed) 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 صلى الله عليه وآله وسلم‎ being inspired Divinely on a very cold day and noticed the Sweat dropping from his forehead (as the Inspiration was over).