About eight days after Jesus said this, he took Peter, John and James with him and went up onto a mountain to pray. As he was praying, the appearance of his face changed, and his clothes became as bright as a flash of lightning. Two men, Moses and Elijah, appeared in glorious splendor, talking with Jesus. They spoke about his departure, which he was about to bring to fulfillment at Jerusalem. 32 Peter and his companions were very sleepy, but when they became fully awake, they saw his glory and the two men standing with him. As the men were leaving Jesus, Peter said to him, “Master, it is good for us to be here. Let us put up three shelters—one for you, one for Moses and one for Elijah.” (He did not know what he was saying.)
While he was speaking, a cloud appeared and covered them, and they were afraid as they entered the cloud. A voice came from the cloud, saying, “This is my Son, whom I have chosen; listen to him.” When the voice had spoken, they found that Jesus was alone. The disciples kept this to themselves and did not tell anyone at that time what they had seen.
If I remember well the book of Judith Lieu on Marcion (I will quote it later), she says that the marcionite interpretation was that there is a conflict between Jesus and Moses & Eliah at Oreb.
Note what there is in Mark and Matthew:
After six days Jesus took with him Peter, James and John the brother of James, and led them up a high mountain by themselves. There he was transfigured before them. His face shone like the sun, and his clothes became as white as the light. Just then there appeared before them Moses and Elijah, talking with Jesus.
Peter said to Jesus, “Lord, it is good for us to be here. If you wish, I will put up three shelters—one for you, one for Moses and one for Elijah.”
While he was still speaking, a bright cloud covered them, and a voice from the cloud said, “This is my Son, whom I love; with him I am well pleased. Listen to him!”
When the disciples heard this, they fell facedown to the ground, terrified. But Jesus came and touched them. “Get up,” he said. “Don’t be afraid.” When they looked up, they saw no one except Jesus.
As they were coming down the mountain, Jesus instructed them, “Don’t tell anyone what you have seen, until the Son of Man has been raised from the dead.”
After six days Jesus took Peter, James and John with him and led them up a high mountain, where they were all alone. There he was transfigured before them. His clothes became dazzling white, whiter than anyone in the world could bleach them. And there appeared before them Elijah and Moses, who were talking with Jesus.
Peter said to Jesus, “Rabbi, it is good for us to be here. Let us put up three shelters—one for you, one for Moses and one for Elijah.” (He did not know what to say, they were so frightened.)
Then a cloud appeared and covered them, and a voice came from the cloud: “This is my Son, whom I love. Listen to him!”
Suddenly, when they looked around, they no longer saw anyone with them except Jesus.
Both Mark and Matthew omit both the real cause (in Mcn) that triggers the ignorant question of Peter in Mcn: Peter was not so completely stupid. He had asked that question because all he had seen, when he wakes up from the initial stunning, was a fact prima facie negative for him: that Moses and Elijah separate themselves from Jesus.
To hide this fact, Matthew and Mark omit the detail that Peter and the disciples were sleeping (and therefore, as they slept, they had not fully realized that what was really happening was not a peaceful conversation, but a bitter dispute Jesus versus Moses and Eliah) in order to explain the cause of the embarrassed question of Peter not as a (proto-catholic) reaction to the division of Elijah and Moses from Jesus, but as a simple fear (so Mark) or simple stupidity (so Matthew).
So Peter ''did not know what he said'' because he believed that the conversation between the three was friendly, when on the contrary Jesus disagreed with Elijah and Moses. Peter for his ignorance is a hindrance to the divine will, since he wants to unite what Jesus has divided (by building the three shelters).
With his question, without knowing, Peter is really tempting Jesus. So this is really the first and only Jesus' temptation.
At that point, the voice spoke from heaven to correct Peter, ''just listen to him'' (Jesus). Implicit: not Moses, not Elijah.
The fact that Mark omitted those two particular (the sleep of the disciples, and the farewell of Moses and Elijah from Jesus, cfr Luke 9:33: ''As the men were leaving Jesus...'') betrays his desire to correct Marcion. And so Mark is after Mcn and not the contrary.