Śatapatha Brāhmaṇa ("of a hundred paths") 2.4.2.23

2.4.2.23

athodapātramādāyāvanejayati | asāvavanenikṣvetyeva yajamānasya pitaramasāvavanenikṣveti pitāmahamasāvavanenikṣveti prapitāmahaṃ tadyathā jakṣuṣe 'bhiṣiñcedevaṃ tat

Thereupon he takes the water-pitcher and makes them wash themselves, merely saying, 'N.N., wash thyself!' (naming) the sacrificer's father; 'N.N., wash thyself!' (naming) his grandfather; 'N.N., wash thyself!' (naming) his great-grandfather. Even as one would pour out (water for a guest) when he has taken his meal, so here.