Ἀλεξάνδρου ἀνάβασις 6.19.5

6.19.5

αὐτὸς δὲ ὑπερβαλὼν τοῦ Ἰνδοῦ ποταμοῦ τὰς ἐκβολὰς ἐς τὸ πέλαγος ἀνέπλει, ὡς μὲν ἔλεγεν, ἀπιδεῖν εἴ πού τις χώρα πλησίον ἀνίσχει ἐν τῷ πόντῳ, ἐμοὶ δὲ δοκεῖ, οὐχ ἥκιστα ὡς πεπλευκέναι τὴν μεγάλην τὴν ἔξω Ἰνδῶν θάλασσαν. ἐνταῦθα ταύρους τε σφάξας τῷ Ποσειδῶνι ἀφῆκεν ἐς τὴν θάλασσαν καὶ σπείσας ἐπὶ τῇ θυσίᾳ τήν τε φιάλην χρυσῆν οὖσαν καὶ κρατῆρας χρυσοῦς ἐνέβαλεν ἐς τὸν πόντον χαριστήρια, εὐχόμενος σῶόν οἱ παραπέμψαι τὸν στρατὸν τὸν ναυτικόν, ὅντινα ξὺν Νεάρχῳ ἐπενόει στέλλειν ὡς ἐπὶ τὸν κόλπον τὸν Περσικὸν καὶ τὰς ἐκβολὰς τοῦ τε Εὐφράτου καὶ τοῦ Τίγρητος.

While the fleet was at anchor here, a vicissitude to which the Great Sea is subject occurred, for the tide ebbed and their ships were left on dry ground. This phenomenon, of which Alexander and his followers had no previous experience, caused them no little alarm and greater still was their dismay, when in due course of time the tide advanced, and the hulls of the vessels were floated aloft. Those vessels which it found settled in the soft mud were uplifted without damage, and floated again, nothing the worse for the strain; but as for those vessels which had been left on a drier part of the beach and were not firmly embedded, some on the advance of a massive wave fell foul of each other, while others were dashed upon the strand and shattered in pieces.
Alexander caused these vessels to be repaired as well as circumstances allowed, and despatched men in advance down the river in two boats to explore an island at which the natives informed him he must anchor on his way to the sea. The name of the island was Killouta. When he learned that the island had harbours, was of great extent, and yielded water, he ordered the rest of the fleet to make its way thither, but he himself with the fastest sailing ships advanced beyond the island to see the mouth of the river, and ascertain whether it offered a safe and easy passage out into the open main. When they had proceeded about 200 stadia beyond the island, they descried another which lay out in the sea. Then they returned to the island in the river, and Alexander, having anchored his ships near its extremity, offered sacrifice to those gods to whom, he said, Ammon had enjoined him to sacrifice. On the following day he sailed down to the other island which lay in the ocean, and approaching close to it also, offered other sacrifices to other gods and in another manner. These sacrifices, like the others, he offered under sanction of an oracle given by Ammon. He then advanced beyond the mouths of the river Indus, and sailed out into the great main to discover, as he declared, whether any land lay anywhere near in the sea, but, in my opinion, chiefly that it might be said that he had navigated the great outer sea of India. He then sacrificed bulls to the god Poseidon, which he threw into the sea; and following up the sacrifice with a libation, he threw the goblet and bowls of gold into the bosom of the deep as thanks-offerings, beseeching the god to conduct in safety the naval expedition which he intended to despatch under Nearchos to the Persian Gulf and the mouths of the Euphrates and Tigris.