Pausanias, Description of Greece 5.14.8
Pausanias guides us round the altars at Olympia.
τὰ δὲ ἐς τὸν μέγαν βωμὸν ὀλίγῳ μέν τι ἡμῖν πρότερόν ἐστιν εἰρημένα, καλεῖται δὲ Ὀλυμπίου Διός. πρὸς αὐτῷ δέ ἐστιν Ἀγνώστων Θεῶν βωμὸς καὶ μετὰ τοῦτον Καθαρσίου Διὸς καὶ Νίκης καὶ αὖθις Διὸς ἐπωνυμίαν Χθονίου. εἰσὶ δὲ καὶ θεῶν πάντων βωμοὶ καὶ Ἥρας ἐπίκλησιν Ὀλυμπίας, πεποιημένος τέφρας καὶ οὗτος· Κλυμένου δέ φασιν αὐτὸν ἀνάθημα εἶναι. μετὰ δὲ τοῦτον Ἀπόλλωνος καὶ Ἑρμοῦ βωμός ἐστιν ἐν κοινῷ, διότι Ἑρμῆν λύρας, Ἀπόλλωνα δὲ εὑρέτην εἶναι κιθάρας Ἑλλήνων ἐστὶν ἐς αὐτοὺς λόγος.
A little earlier I spoke on the subject of the great altar, which is called the altar of Olympian Zeus. Next to it is an altar of Unknown Gods, and after this an altar of Zeus the Purifier, an altar of Victory, and another altar of Zeus with the surname ‘Of The Earth’. And there are altars of all gods, and of Hera with the surname Olympian, which is also [i.e. like that of Olympian Zeus] made of ashes. They say that it was dedicated by Clymenus. After this is the altar of Apollo and Hermes, which they share because the Greeks’ story about them is that Hermes was the inventor of the lyre and Apollo the inventor of the kithara.