Diodorus of Sicily: Greek historian, author of the Library of World History. His activities can be dated between 60 and 30 BCE.
In November 333, Alexander the Great and his trusted general Parmenion defeated the Persian king Darius III Codomannus on the uneven coastal plain south of Issus. A full description of this battle can be found here. There are many sources on this event. The Greek author Diodorus of Sicily describes the turning point: the moment of Darius' retreat. This is the moment that is depicted on the Alexander mosaic.
The translation of section 17.34 of the World History was made by C. Bradford Welles.
Diodorus on the Battle of Issus
[17.34.2] The Persian Oxyathres was the brother of Darius and a man highly praised for his fighting qualities. When he saw Alexander riding at Darius and feared that he would not be checked, he was seized by the desire to share his brother's fate.
[17.34.3] Ordering the best of the horsemen in his company to follow him, he threw himself with them against Alexander, thinking that this demonstration of brotherly love would bring him high renown among the Persians. He took up the fight directly in front of Darius' chariot and there, engaging the enemy skillfully and with a stout heart slew many of them.
[17.34.4] The fighting qualities of Alexander's group were superior, however, and quickly many bodies lay piled high about the chariot. No Macedonian had any other thought than to strike the king, and in their intense rivalry to reach him took no thought for their own lives.
[17.34.5] Many of the noblest Persian princes perished in this struggle, among them Atizyes and Rheomithres and Sabaces, the satrap of Egypt.note[Rheomithres had been cavalry commander during the battle on the banks of the river Granicus in 334.] Many of the Macedonians fell also, and Alexander was wounded in the thigh, for the enemy pressed about him.
[17.34.6] The horses which were harnessed to the yoke of Darius' chariot were covered by wounds and terrified by the piles of dead about them. They refused to answer to their bridles, and came close to carrying off Darius into the midst of the enemy, but the king himself, in extreme peril, caught up the reigns, being forced to throw away the dignity of his position and to violate the ancient custom of the Persian kings.
[17.34.7] A second chariot was brought up by Darius' attendants and in the confusion - as he changed over to it in the face of constant attack - he fell into a panic terror. Seeing their king in this state, the Persians with him turned to flee.