Then Peter said to Jesus, "Rabbi, it is good for us to be here! Let us make three tabernacles: one for You, one for Moses, and one for Elijah," because he did not know what he should say, since they were terrified. A cloud appeared, overshadowing them, and a voice came from the cloud, "This is My beloved Son; listen to Him!" - Mark 9:5-7
At the sight of a transfigured Jesus (along with a glorified Moses and Elijah), Peter apparently struggled to think of something to say and just blurted out what immediately came to mind. God replied to Peter's suggestion (to build three tabernacles) by emphatically reminding Peter of the inequality of the three men who stood before him.
Moses (who represented the Law) and Elijah (who represented the Prophets) found their fulfillment in Christ; therefore, God directed Peter to an understanding of the Law and Prophets through the lens of Christ.
God was also saying more. He intended Peter to realize that some experiences in life do not need commentary and we are better served by listening. There are times when we must be still and silent in order to know that He is God.
Never miss a good chance to shut up. ― Will Rogers
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