Warnings about false prophets and false teachers are common in the Old and the New Testament. Here are a few samples:
But the prophet who presumes to speak a word in My name that I have not commanded him to speak, or who speaks in the name of other gods, that same prophet shall die (Deuteronomy 18:20).
Many false prophets will arise and lead many astray (Matthew 24:11).
False prophets also arose among the people, just as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Master who bought them, bringing upon themselves swift destruction. And many will follow their sensuality, and because of them the way of truth will be blasphemed (2 Peter 2:1-2).