Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.4.14

2.2.4.14

tadvā etadevaitāsāṃ nāma | etadyajñasya tasmādetatpariharetsādhu puṇyamiti bahvyo ha vā asyaitā bhavantyupanāmuka enaṃ yajño bhavati ya evaṃ vidvānetatpariharati sādhu puṇyamiti

This (word 'go'), then, is a name of those (cows), and so it is of the sacrifice: let him, therefore, repeat it, (as it were) saying, 'Good, excellent!' and, verily, whosoever, knowing this, repeats it; (as it were) saying, 'Good, excellent!' with him those (cows) multiply, and the sacrifice will incline to him.