Arrian

“ Both armies stood a long time at the margin of the river, keeping quiet from dread of the result; and profound silence was observed on both sides. For the Persians were waiting till the Macedonians should step into the water, with the intention of attacking them as they emerged. ”

“ Alexander leaped upon his steed, ordering those about him to follow, and exhorting them to show themselves valiant men. He then commanded Amyntas, son of Arrhabaeus, to make the first rush into the river at the head of the skirmishing cavalry, the Paeonians, and one regiment of infantry; and in front of these he had placed Ptolemy, son of Philip, in command of the squadron of Socrates, which body of men indeed on that day happened to have the lead of all the cavalry force. ”