The Hadiths Sahih Al-Bukhari - Vol 1 Book 1 Number 3
You. Volume One book One, number Three narrated aisha the Mother of the Faithful Believers the commencement of the Divine inspiration to Allah's Apostle was in the form of good dreams, which came true like bright daylight, and then the love of seclusion was bestowed upon him. He used to go in seclusion in the cave of Hira, where he used to worship Allah alone continuously for many days before his desire to see his family