Sahih Bukhari
Hadith # 4

Period Of Pause In The Revelation 

Translation:

 Hazrat Jabir bin Abdullah al-Ansari narrates that the Prophet (Peace and blessings of Allah be upon Him) told the time of pause in the revelation and said, "I was walking when all of a sudden I heard a voice from the sky  and looked up." Seeing the angel who had visited me on a chair between heaven and earth in the cave of Hira, I got scared and returned home and said; "Wrap me (in a blanket). And then Allah revealed the following Holy verses (of the Qur'an): 'O you (i.e. Muhammad) wrapped in clothes!' Recite and warn (people against Allah's Punishment),... up to 'and desert the idols.' (74.1-5).

Then the revelation begins to come strongly, frequently, and regularly."