Śatapatha Brāhmaṇa ("of a hundred paths") 3.9.3.30

3.9.3.30

taddhaike | eva maitrāvaruṇacamase vasatīvarīrnayantyā maitrāvaruṇacamasādvasatīvarīṣu yaścāsau pūrvedyurāhṛto yajñasya raso yaścādyāhṛtastamevaitadubhayaṃ saṃsṛjāma iti vadantastadu tathā na kuryādyadvā ādhavanīye samavanayati tadevaiṣa ubhayo yajñasya rasaḥ saṃsṛjyate 'tha hotṛcamase vasatīvarīrgṛhṇāti nigrābhyābhyastadyaduparyupari cātvālaṃ saṃsparśayatyato vai devā divamupodakrāmaṃstadyajamānamevaitatsvargyam panthānamanusaṃkhyāpayati

Now some indeed pour (some of) the Vasatîvarî water into the Maitrâvaruna's cup, and from the Maitrâvaruna's cup (back) to the Vasatîvarî water, arguing, 'Thereby we mix together both the sap of the sacrifice which was fetched yesterday and that fetched to-day.' But let him not do this; for when he pours (the water) together into the Âdhavanîya trough, then both kinds of sap are mixed together. Thereupon he pours the Vasatîvarî water into the Hotri's cup for the Nigrâbhyâs. And as to why he makes them touch one another close over the pit, it was from thence, forsooth, that the gods rose to heaven; he thus makes the sacrificer look along the road to heaven.