Passage 1.27.7
τῶν δὲ ἐν Τροιζῆνι λόγων, οὓς ἐς Θησέα λέγουσιν, ἐστὶν ὡς Ἡρακλῆς ἐς Τροιζῆνα ἐλθὼν παρὰ Πιτθέα καταθεῖτο ἐπὶ τῷ δείπνῳ τοῦ λέοντος τὸ δέρμα, ἐσέλθοιεν δὲ παρʼ αὐτὸν ἄλλοι τε Τροιζηνίων παῖδες καὶ Θησεὺς ἕβδομον μάλιστα γεγονὼς ἔτος· τοὺς μὲν δὴ λοιποὺς παῖδας, ὡς τὸ δέρμα εἶδον, φεύγοντάς φασιν οἴχεσθαι, Θησέα δὲ ὑπεξελθόντα οὐκ ἄγαν σὺν φόβῳ παρὰ τῶν διακόνων ἁρπάσαι πέλεκυν καὶ αὐτίκα ἐπιέναι σπουδῇ, λέοντα εἶναι τὸ δέρμα ἡγούμενον.
Among the stories told in Troezen about Theseus, they say that when Heracles came to Troezen as a guest of Pittheus, he laid down the lion's skin during dinner. Then some of the Troezenian children entered the room, including Theseus, who was just reaching his seventh year. All the other boys, on seeing the skin, fled in fear, but Theseus, slipping out quietly and without great fright, seized an axe from one of the attendants and quickly returned toward it, believing the lion skin to be a living lion.