Vishnupurana 3.18.98

3.18.98

kriyāhānirgṛhe yasya māsamekaṃ prajāyate /
tasyāvalokanātsūryaṃ paśyeta matimānnaraḥ //

Such, Maitreya, is the sin of conversing with a heretic, and such are the expiatory effects of bathing after the solemn sacrifice of a horse, as I have narrated them to you. Let therefore a man carefully avoid the discourse or contact of an unbeliever, especially at seasons of devotion, and when engaged in the performance of religious rites preparatory to a sacrifice. If it be necessary that a wise man should look at the sun, after beholding one who has neglected his domestic ceremonies for a month, how much greater need must there be of expiation after encountering one who has wholly abandoned the Vedas? one who is supported by infidels, or who disputes the doctrines of holy writ? Let not a person treat with even the civility of speech, heretics, those who do forbidden acts, pretended saints, scoundrels, sceptics, and hypocrites. Intercourse with such iniquitous wretches, even at a distance, all association with schismatics, defiles; let a man therefore carefully avoid them.