Islamic tradition holds that Muhammad, then about forty years old and in the habit of retreating to meditate in the Cave of Hira on a mountain near Mecca, was visited there by the angel Jibril, who commanded him to recite and, when Muhammad protested that he could not, embraced him until he spoke the words later recognized as the opening verses of the Quran's ninety sixth chapter. The earliest surviving full biography of Muhammad, compiled by Ibn Ishaq in the mid-eighth century and preserved only through a later recension by Ibn Hisham, together with hadith collections such as Sahih al-Bukhari, supplies the narrative detail beyond the bare fact of revelation: Muhammad's fear on returning home, his wife Khadija's reassurance and her covering him to calm him, and the confirmation of her Christian kinsman Waraqa ibn Nawfal that what had come to Muhammad matched the pattern by which earlier prophets, including Moses, had received revelation. These sources were compiled more than a century after the events they describe and rely on chains of named transmitters, the isnad, that the Islamic tradition itself developed as a method of authentication.
Ibn Ishaq's work does not survive in its original form; what is read today is a recension by Ibn Hisham, who states in his own introduction that he omitted some material his source had included, so the Sira available now is already a filtered version of an already-late source, a point specialists in early Islamic history weigh when assessing how much of the narrative detail can be traced to the earliest layer of transmission.