Strabo, a geographer of the first century B.C.E., writes:

And they recount this tradition about Pheidias. When Panaenus asked him what model he intended to employ in making the image of Zeus, he replied that it was the model provided by Homer in the following lines of the Iliad (1.528-530): "Thus spoke the son of Kronos and nodded his dark brow, and the ambrosial locks flowed down from the lord's immortal head, and he made great Olympus shake"

Source: Strabo, 8 3 30