Diodorus Siculus 17.32
 Alexander's mother wrote at this time to him, giving him other useful advice and warning him to be on his guard against the Lyncestian Alexander.This was a man distinguished for bravery and high spirit who accompanied the king in the group of Friends in a trusted capacity.  There were many other plausible circumstances joining to support the charge, and so the Lyncestian was arrested and bound and placed under guard, until he should face a court.
Alexander learned that Dareius was only a few days march away, and sent off Parmenion with a body of troops to seize the passage of the so‑called . . . Gates. When the latter reached the place, he forced out the Persians who were holding the pass and remained master of it.  Dareius decided to make his army mobile and diverted his baggage train and the non-combatants to Damascus in Syria; then, learningthat Alexander was holding the passes and thinking that he would never dare to fight in the plain, made his way quickly to meet him.  The people of the country, who had little respect for the small numbers of the Macedonians but were much impressed with the great size of the Persian army, abandoned Alexander and came over to Dareius. They brought the Persians food and other materials with great goodwill, and mentally predicted victory for them. Alexander, however, occupied Issus, a considerable city, which was terrified into submission.