"Sahih Al-Bukhari Hadith 9.111 Narrated by Aisha
The commencement of the Divine Inspiration to Allah's Apostle was in the form of good righteous dreams in his sleep. He never had a dream but that it came true like bright day light. He used to go in seclusion (the cave of) Hira where he used to worship (Allah Alone) continuously for many (days) nights.

The angel came to him in (Hira) and asked him to read. The Prophet replied, "I do not know how to read."
(The Prophet added), "The angel caught me (forcefully) and pressed me so hard that I could not bear it anymore. He then released me and again asked me to read, and I replied, "I do not know how to read," whereupon he caught me again and pressed me a second time till I could not bear it anymore.
He then released me and asked me again to read, but again I replied, "I do not know how to read Thereupon he caught me for the third time and pressed me and then released me and said,