Tradition
St. Irenaeus
Fragments from the Lost Writings of Irenaeus
Chapter 23
“And he mounted upon his ass.” The ass was the type of the body of Christ, upon whom all men, resting from their labours, are borne as in a chariot. For the Saviour has taken up the burden of our sins. Now the angel who appeared to Balaam was the Word Himself; and in His hand He held a sword, to indicate the power which He had from above.
Source: Fragments from the Lost Writings of Irenaeus (New Advent)