Jamieson, Fausset and Brown on Numbers 16:15

15. Moses was very wroth--Though the meekest of all men [Nu 12:3], he could not restrain his indignation at these unjust and groundless charges; and the highly excited state of his feeling was evinced by the utterance of a brief exclamation in the mixed form of a prayer and an impassioned assertion of his integrity. (Compare 1Sa 12:3).

and said unto the Lord, Respect not thou their offering--He calls it their offering, because, though it was to be offered by Korah and his Levitical associates, it was the united appeal of all the mutineers for deciding the contested claims of Moses and Aaron.