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

Sahih Bukhari
Volume 1, Book 1, Number 2