Rejoice heart and soul, daughter of Zion! Shout with gladness, daughter of Jerusalem! See now, your king comes to you; he is victorious, he is triumphant, humble and riding on a donkey, on a colt, the foal of a donkey (Zechariah, 9.9).
The picture represent Assisi fresco by Pietro Lorenzetti fragment from Lower Basilica, southern transept, see link
http://diglib.library.vanderbilt.edu/act-imagelink.pl?RC=55432