Jamieson, Fausset and Brown on 2 Peter 2:16

16. was rebuked--Greek, "had a rebuke," or conviction; an exposure of his specious wickedness on his being tested (the root verb of the Greek noun means to "convict on testing").

his--Greek, "his own": his own beast convicted him of his own iniquity.

ass--literally, "beast of burden"; the ass was the ordinary animal used in riding in Palestine.

dumb--Greek, "voiceless-speaking in man's voice"; marking the marvellous nature of the miracle.

forbade--literally, "hindered." It was not the words of the ass (for it merely deprecated his beating it), but the miraculous fact of its speaking at all, which withstood Balaam's perversity in desiring to go after God had forbidden him in the first instance. Thus indirectly the ass, and directly the angel, rebuked his worse than asinine obstinacy; the ass turned aside at the sight of the angel, but Balaam, after God had plainly said, Thou shalt not go, persevered in wishing to go for gain; thus the ass, in act, forbade his madness. How awful a contrast--a dumb beast forbidding an inspired prophet!