Plutarch Antony 41

[1] As he was about to lead his army back by the road over which it had come, which ran through a level country without trees, a man of the Mardian race, who had great familiarity with the Parthian habits, and had already shown himself faithful to the Romans in the battle over the engines of war, ¹ came to Antony and urged him in his flight to keep close to the hills upon his right, and not to expose an encumbered army of legionaries to so large a force of mounted archers, in bare and extended tracts;

[2] this was the very thing, he said, which Phraates had designed when he induced him by friendly conferences to raise the siege; he himself, he said, would conduct the army by a way that was shorter and furnished a greater abundance of provisions.
  On hearing this, Antony took counsel with himself. He did not wish to have the appearance of distrusting the Parthians, now that a truce had been made, but since he approved of the shorter road and of having their march take them past inhabited villages, he asked the Mardian for a pledge of his good faith.

[3] The Mardian offered to let himself be put in fetters until he should bring the army safely into Armenia, and he was put in fetters, and led them for two days without their encountering trouble. But on the third day, when Antony had put the Parthians entirely out of his thoughts, and was marching along in loose order because of his confidence, the Mardian noticed that a dike of the river had been recently torn away, and that the stream was flowing out in great volume towards the road over which their march must be made.

[4] He comprehended that this was the work of the Parthians, throwing the river in their way to obstruct and delay the Roman march, and urged Antony to look out and be on his guard, as the enemy were near. And just as Antony was setting his legionaries in array and arranging to have his javelineers and slingers make a sally through them against the enemy, the Parthians came into view and began to ride around the army in order to envelope and throw it into confusion on all sides.

[5] Whenever the Roman light-armed troops sallied out against them, the Parthians would inflict many wounds with their arrows, but sustain yet more from the leaden bullets and javelins of the Romans, and therefore withdraw. Then they would come up again, until the Celts, massing their horses together, made a charge upon them and scattered them, so that they showed themselves no more that day.