Ammianus Marcellinus Res Gestae 23.3.6

[6] After these arrangements had thus been made, he himself feigned a march across the Tigris, an expedition for which he had also ordered supplies to be carefully prepared, but then turned to the right and, after passing a quiet night, called next morning for the mount which he usually rode. And when the horse, called Babylonius, was brought to him, it was laid low by a missile from the artillery, and as it rolled on the ground in unbearable pain, it scattered about its ornaments, which were adorned with gold and precious stones. Delighted by this omen, Julian cried out amid expressions of joy from the bystanders, that Babylon had fallen to the ground, stripped of all its adornments.