Śatapatha Brāhmaṇa ("of a hundred paths") 3.3.4.31

3.3.4.31

atra haike | udapātramupaninayanti yathā rājña āgatāyodakamāharedevametaditi vadantastadu tathā na kuryānmānuṣaṃ ha te yajñe kurvanti vyṛddhaṃ vai tadyajñasya yanmānuṣaṃ nedvyṛddhaṃ yajñe karavāṇīti tasmānnopaninayet

Some now pour out a vessel of water beside him, saying that this would be as. one would bring water for a king that has come to him. But let him not do this, for they (who do this) do at the sacrifice what is human, and inauspicious for the sacrifice forsooth is that which is human: let him therefore not pour out water, lest he should do at the sacrifice what is inauspicious.