Passage 5.15.1
ἔστι δὲ οἴκημα ἐκτὸς τῆς Ἄλτεως, καλεῖται δὲ ἐργαστήριον Φειδίου, καὶ ὁ Φειδίας καθʼ ἕκαστον τοῦ ἀγάλματος ἐνταῦθα εἰργάζετο· ἔστιν οὖν βωμὸς ἐν τῷ οἰκήματι θεοῖς πᾶσιν ἐν κοινῷ. ὀπίσω δὲ ἀναστρέψαντι αὖθις ἐς τὴν Ἄλτιν ἐστὶν ἀπαντικρὺ τοῦ Λεωνιδαίου---
Outside the Altis there is a building known as the Workshop of Pheidias, where Pheidias himself fashioned each part of the statue. Within the building there is an altar dedicated jointly to all the gods. Turning back again towards the Altis, opposite is the Leonidaeum.