A - Thanks for the question. Here is one of the passages that fits what you are describing:
"Now there was a man full of leprosy in one of the towns where he was; and when he saw Jesus, he fell prostrate, pleaded with him, and said, "Lord, if you wish, you can make me clean." Jesus stretched out his hand, touched him, and said, "I do will it. Be made clean." And the leprosy left him immediately. Then he ordered him not to tell anyone, but "Go, show yourself to the priest and offer for your cleansing what Moses prescribed; that will be proof for them." The report about him spread all the more, and great crowds assembled to listen to him and to be cured of their ailments, but he would withdraw to deserted places to pray." - Luke 5: 12-16Jesus tell the people who witness His miracles (and exorcisms and professions of His mission) are told not to tell others frequently. This is sometimes called the "messianic secret". It is a pretty common theme in the Gospels, especially in the Gospel of Mark.
To understand what was going on, we have to understand how the Jews viewed the Messiah 2,000 years ago. They thought the Messiah would be a political and military hero who would free Israel from the Romans. This kind of expectation permeated the people in Jesus day - and Jesus didn't fulfill this expectation.
Christ came to set us free from sin and death - not the Romans! So, he asked others to keep quiet so He could teach the people not to expect a military or political leader, but a spiritual one.
Some others say that he didn't want to inflame his enemies or that he wanted to avoid being seen as just a miracle-worker. Both of these are also valid points, but I believe the messianic secret makes the most sense in reference to why Jesus would ask others to keep quiet.
I hope this helps.