Agnipurana 168.8

168.8

brāhmaṇānnañca śūdreṇa nādyāccaiva nimantritaḥ /
eṣāmanyatamasyānnamamatyā vā tryahaṃ kṣapet //

And the food of a brāhmaṇa should not be eaten by a śūdra, even when invited. / If one eats the food of any of these unwittingly, one should fast for three days;