According to the Islamic tradition, in 610, while sitting in the cave of Hira he had come to many times before, Muhammad ﷺ experienced something new. An angel suddenly appeared to him in the cave, commanding him, “Read!” He responded that he did not know how. Again, the angel demanded he read. Again, Muhammad ﷺ responded that he was unable to. A third time, the angel demanded he read, and for a third time, Muhammad ﷺ responded that he was unable to. The angel then recited to him the first verses of the Quran to be revealed (Quran 96).
Source: Lost Islamic History (Pg. 10-11)