Who says Jesus warned us about being deceived by false teachers?

Answer 1

Jesus spoke about false teachers/prophets:

Matthew 7:15 Beware of false prophets, who come to you in sheep's clothing, but inwardly they are ravenous wolves.

Matthew 24:11 Then many false prophets will rise up and deceive many.

Paul spoke about false teachers:

2 Corinthians 11:13 For such are false apostles, deceitful workers, transforming themselves into apostles of Christ.

And

Acts 20:29 For I know this, that after my departure savage wolves will come in among you, not sparing the flock.

Acts 20:30 Also from among yourselves men will rise up, speaking perverse things, to draw away the disciples after themselves.

Peter spoke about false teachers

2 Peter 2:1 But there were also false prophets among the people, even as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Lord who bought them, and bring on themselves swift destruction.