Passage 5.13.11
κατʼ ἔτος δὲ ἕκαστον φυλάξαντες οἱ μάντεις τὴν ἐνάτην ἐπὶ δέκα τοῦ Ἐλαφίου μηνὸς κομίζουσιν ἐκ τοῦ πρυτανείου τὴν τέφραν, φυράσαντες δὲ τῷ ὕδατι τοῦ Ἀλφειοῦ κονιῶσιν οὕτω τὸν βωμόν. ὑπὸ δὲ ἄλλου τὴν τέφραν ὕδατος ποιηθῆναι πηλὸν οὐ μή ποτε ἐγγένηται· καὶ τοῦδε ἕνεκα ὁ Ἀλφειὸς νενόμισται τῷ Ὀλυμπίῳ Διὶ ποταμῶν δὴ μάλιστα εἶναι φίλος. ἔστι δὲ καὶ ἐν Διδύμοις τῶν Μιλησίων βωμός, ἐποιήθη δὲ ὑπὸ Ἡρακλέους τοῦ Θηβαίου, καθὰ οἱ Μιλήσιοι λέγουσιν, ἀπὸ τῶν ἱερείων τοῦ αἵματος· ἐς δὲ τὰ ὕστερα τὸ αἷμα τῶν θυμάτων οὐκ ἐς ὑπέρογκον ηὔξηκεν αὐτὸν μέγεθος.
Each year, on the nineteenth day of the month Elaphius, the seers take ashes from the Prytaneion, mix them with water from the Alpheius river, and thus plaster the altar. It is never permitted that this ash be mixed into mud with water from another. For this reason, the Alpheius has come to be regarded as the river most dear to Olympian Zeus. There is also at Didyma among the Milesians an altar they say was constructed by Heracles of Thebes, made from the blood of sacrificial animals. The blood of sacrifices offered afterward, however, has not increased the altar's size significantly.