Shatapatha Brahmana 4.3.1.21

4.3.1.21

atha pratiprasthātābhakṣitena pātreṇa | aindrāgnaṃ grahaṃ gṛhṇāti tadyadabhakṣitena pātreṇaindrāgnaṃ grahaṃ gṛhṇāti na vā ṛtugrahāṇāmanuvaṣaṭkurvantyetebhyo vā aindrāgnaṃ grahaṃ grahīṣyanbhavati tadasyaindrāgnenaivānuvaṣaṭkṛtā bhavanti

Thereupon the Pratiprasthâtri draws the Aindrâgna graha with the vessel not used for the drinking. The reason why he draws the Aindrâgna libation with the vessel not used for drinking is that no second Vashat is pronounced on the Ritugrahas, and for them he is about to take the Aindrâgna graha: thus they become consecrated for him by a second Vashat through the Aindrâgna.