Matthew 24:4-5 (KJV)
"Take heed that no man deceive you. For many shall come in my name, saying, I am Christ; and shall deceive many."
Jesus continues this warning a few verses later:
Matthew 24:11 — Many false prophets will arise and mislead many. Bible Hub
And again in verse 24:
Matthew 24:24 — "False christs and false prophets will rise and show great signs and wonders to deceive, if possible, even the elect." United Church of God
There's also a related verse from the Sermon on the Mount:
Matthew 7:15 — "Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravenous wolves."