Passage 1.21.2
τὴν δὲ εἰκόνα τὴν Αἰσχύλου πολλῷ τε ὕστερον τῆς τελευτῆς δοκῶ ποιηθῆναι καὶ τῆς γραφῆς ἣ τὸ ἔργον ἔχει τὸ Μαραθῶνι. ἔφη δὲ Αἰσχύλος μειράκιον ὢν καθεύδειν ἐν ἀγρῷ φυλάσσων σταφυλάς, καί οἱ Διόνυσον ἐπιστάντα κελεῦσαι τραγῳδίαν ποιεῖν· ὡς δὲ ἦν ἡμέρα--- πείθεσθαι γὰρ ἐθέλειν---ῥᾷστα ἤδη πειρώμενος ποιεῖν.
The portrait of Aeschylus, I believe, was made much later than his death and later than the painting showing the battle at Marathon. Aeschylus, it is said, while still a youth, fell asleep as he kept watch over grape clusters in a field. Dionysus appeared, standing beside him, and commanded him to compose tragedy. At daybreak—for Aeschylus was eager to obey—he immediately set about attempting to compose it.