Karen Armstrong writes engagingly about Islam and its prophet. Here's how she describes the first vision:
AFTERWARDS HE FOUND IT almost impossible to describe the experience that sent him running in anguish down the rocky hillside to his wife. It seemed to him that a devastating presence had burst into the cave where he was sleeping and gripped him in an overpowering embrace, squeezing all the breath from his body. In his terror, Muhammad could only think that he was being attacked by a jinni, one of the fiery spirits who haunted the Arabian steppes and frequently lured travellers from the right path. The jinn also inspired the bards and soothsayers of Arabia. One poet described his poetic vocation as a violent assault: his personal jinni had appeared to him without any warning, thrown him to the ground and forced the verses from his mouth.1 So, when Muhammad heard the curt command “Recite!” he immediately assumed that he too had become possessed. “I am no poet,” he pleaded. But his assailant simply crushed him again, until—just when he thought he could bear it no more—he heard the first words of a new Arabic scripture pouring, as if unbidden, from his lips.
He had this vision during the month of Ramadan, 610 CE. Later Muhammad would call it layla al-qadr (the “Night of Destiny”) because it had made him the messenger of Allah, the high god of Arabia. But at the time, he did not understand what was happening. He was forty years old, a family man, and a respected merchant in Mecca, a thriving commercial city in the Hijaz. Like most Arabs of the time, he was familiar with the stories of Noah, Lot, Abraham, Moses, and Jesus and knew that some people expected the imminent arrival of an Arab prophet, but it never occurred to him that he would be entrusted with this mission. Indeed, when he escaped from the cave and ran headlong down the slopes of Mount Hira’, he was filled with despair. How could Allah have allowed him to become possessed? The jinn were capricious; they were notoriously unreliable because they delighted in leading people astray. The situation in Mecca was serious. His tribe did not need the dangerous guidance of a jinni. They needed the direct intervention of Allah, who had always been a distant figure in the past, and who, many believed, was identical with the God worshipped by Jews and Christians.