The Prophet PBUH said,

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."

Hadrat 'Aisha the mother of the faithful believers 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).  

Islam is based on (the following) five (principles):

-   To testify that none has the right to be worshipped but Allah and Muhammad is Allah's Apostle.

-   To offer the (compulsory congregational) prayers dutifully and perfectly.

-   To pay Zakat (i.e. obligatory charity) .

-   To perform Hajj. (i.e. Pilgrimage to Mecca)

-   To observe fast during the month of Ramadan.

Narrated Ibn 'Umar: Bukhari