Anushasana Parva 104.92

Having done this, one should perforin the adoration of the departed Manes and celestials, according to the ordinances of Vedas. Listen to me, O you of Kuru's race, as I tell you what purification is cleansing and beneficial for a Brahmana. Before beginning to eat and after finishing the meal, and in all deeds requiring purification, the Brahmana should rinse his mouth with water placed on the limb called the Brahmatirtha.