The angel who had first come to him in the cave of Hira’ appears again, seated between earth and sky. Shaken, he hurries home asking to be covered, and the opening verses of Surah al-Muddaththir are revealed: “Arise and warn.” After this, revelation comes regularly.
As the book dates it: at about forty (the start of Prophethood)
Arise and warn! And your Lord magnify!