Śatapatha Brāhmaṇa ("of a hundred paths") 2.4.2.16

2.4.2.16

athodapātramādāyāvanejayati | asāvavanenikṣvetyeva yajamānasya pitaramasāvavanenikṣveti pitāmahamasāvavanenikṣveti prapitāmahaṃ tadyathāśiṣyate 'bhiṣiñcedevaṃ tat

He then takes the water-pitcher and makes (the fathers) wash (their hands), 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. As one would pour out water (for a guest) when he is about to take food, so in this case.