Anabasis

Arrian

The route is desolate; most of it is sand, and waterless. Alexander, however, had plenty of rain, and this was attributed to the divinity. And so was the following incident. Whenever a south wind blows in that country, it makes a great heap of sand on the route and obscures its marks, and one cannot get one’s bearings in a sort of ocean of sand, since there are no marks along the route, no mountain anywhere, no tree, no solid hillocks standing up, by which the wayfarers might judge their proper course, as sailors do from the stars; in fact Alexander’s army went astray, and the guides were in doubt as to the route. Now Ptolemy son of Lagos says that two serpents preceded the army giving voice, and Alexander told his leaders to follow them and trust the divinity; and the serpents led the way to the oracle and back again. But Aristobulus agrees with the more common and prevalent version, that two crows, flying in advance of the army, acted as guides to Alexander. That some divine help was given him I can confidently assert, because probability suggests it too; but the exact truth of the story cannot be told; that is precluded by the way in which different writers about Alexander have given different accounts. — 3.3.4-6