Mark 9:9 - Now as they came down from the mountain, He commanded them that they should tell no onethe things they had seen, till the Son of Man had risen from the dead.
Jesus didn't command them not to tell anyone what they had seen; He commanded them not to tell anyone YET. It was quite common for Jesus to teach his "inner circle" something that they were not to proclaim until later; after either His resurrection or ascension.
John 16:4 - But these things I have told you, that when the time comes, you may remember that I told you of them. And these things I did not say to you at the beginning, because I was with you.
It's also quite easy to understand why Jesus sometimes told people to remain silent about His miracles; sensationalism could actually hinder His ministry, so He occasionally took steps to avoid it. There's nothing "strange" about it. It was simply a matter of prudence.
[Quotes from NKJV]
This is the transfiguration, when Peter, James and John saw Jesus change his appearance and then saw him taking to Elijah and Moses.
There is no reason given in The Bible for this strange request. However, it is a feature of Mark's Gospel that Jesus asked people not to tell others, when they witnessed a miracle.
Another view, held by some scholars, is that if no one had heard about the transfiguration when Mark's Gospel was written, then the author's credibility might have been questioned. The author could avoid this by writing that the disciples, the only witnesses to the transfiguration, were warned by Jesus not to tell anyone else until after he had risen. Jesus' permission to tell others after the resurrection would then be seen as fulfilled by the gospel itself.
Elijah and Moses
Moses and Elijah appeared to him.
The names of the prophets dad appeared with Jesus were Moses and Elijah.
Moses and Elijah appeared on the mount of transfiguration with Jesus ans a few of the disciples. Moses stood for the law and Elijah stood for the prophets, it was the near end of Jesus ministry.
The bible tells us that Moses and Elijah were there with Jesus, but Jesus had also taken Peter, James and John with him.
It sounds as if you are asking about the transfiguration of Jesus. Moses and Elijah both appeared to Jesus on Mt. Tabor during the transfiguration in the presence of Peter, James and John.
Moses and Elijah
The three apostles Peter, James, and John saw Moses and Elijah at the Transfiguration of Jesus detailed in Matthew 17. They see these two at a mountain top where Jesus had taken these apostles.
The question is not clear, who appeared when? It was the Angels at the tomb and Moses and Elijah on the mountain.
He was a prophet whose work began about 875 BC from Tishbe of Gilead in the Northern Kingdom. Malachi promised God would send Elijah the prophet before the coming day of the Lord. John the Baptist was spoken of as the one who would go before the Messiah in the spirit and power of Elijah. Elijah appeared with Moses on the Mount of Transfiguration with Jesus.
On the Mount of Transfiguration, Jesus was "transfigured," meaning that he was seen by the Apostles Peter, James and John in his glorified form, bright as the sun in blazingly white clothes. In this form, he spoke with Moses and Elijah, who also appeared there with Jesus. See Luke 9:28-36.
Jesus is God. John the baptist and Elijah are not.