Hi Kalyanamitta1,
A good reference is the first sutta in the sutta Piṭaka, the Brahmajaala Sutta (https://suttacentral.net/dn1/en/bodhi). Here the Buddha says:
“If, bhikkhus, others speak in dispraise of me, or in dispraise of the Dhamma, or in dispraise of the Sangha, you should not give way to resentment, displeasure, or animosity against them in your heart. For if you were to become angry or upset in such a situation, you would only be creating an obstacle for yourselves. If you were to become angry or upset when others speak in dispraise of us, would you be able to recognize whether their statements are rightly or wrongly spoken?”
“Certainly not, Lord.”
“If, bhikkhus, others speak in dispraise of me, or in dispraise of the Dhamma, or in dispraise of the Sangha, you should unravel what is false and point it out as false, saying: ‘For such and such a reason this is false, this is untrue, there is no such thing in us, this is not found among us.”
On a personal note, I rarely tend to get angry towards such people but feel something like “can’t these people understand this, are they this foolish”. In that instance I contemplate that I too would have been similar to those people in the past. Certainly before I came across purdhamma and also in innumerable past lives. So that way I can put myself in their shoes and this lets me develop compassion towards them. Like the metta bhavana that Lal mentioned.
Most importantly I look at myself and think if an Arahanth saw me they would think that I am foolish as well, for being engrossed in these sense pleasures and not understanding the true aniccha, dukka, anatta nature of the world. This helps me a lot.