Albert Barnes Commentary Numbers 20:11-12

"And Moses lifted up his hand, and smote the rock with his rod twice: and water came forth abundantly, and the congregation drank, and their cattle. And Jehovah said unto Moses and Aaron, Because ye believed not in me, to sanctify me in the eyes of the children of Israel, therefore ye shall not bring this assembly into the land which I have given them." — Numbers 20:11-12 (ASV)

The command (Numbers 20:8) was Speak ye unto the rock. The act of striking it, and especially with two strokes, indicates violent irritation on Moses's part, as does his unseemly way of addressing the people: Hear now, ye rebels.

The form of the question, too, must we, etc., directs the people not to God as their deliverer, as ought to have been the case, but to Moses and Aaron personally.

In fact, the faithful servant of God, worn out by the reiterated perversities of the people, breaks down; and in the actual discharge of his duty as God’s representative before Israel, acts unworthily of the great function entrusted to him.

Thus, Moses did not sanctify God in the eyes of the children of Israel. Aaron might have checked the intemperate words and acts of Moses, and did not.

Therefore, God punishes both by withdrawing them from their work for Him and handing over its accomplishment to another.