Śatapatha Brāhmaṇa ("of a hundred paths") 5.1.2.16

5.1.2.16

atha yatra pūrvayā dvārā | vasatīvarīḥ prapādayanti tadaparayā dvārā neṣṭā parisrutam prapādayati dakṣiṇataḥ pātrāṇyabhyavaharanti puro 'kṣameva pratyaṅṅāsīno 'dhvaryuḥ somagrahāngṛhṇāti paścādakṣam prāṅāsīno neṣṭā surāgrahāntsomagrahamevādhvaryurgṛhṇāti surāgrahaṃ neṣṭā somagrahamevādhvaryurgṛhṇāti surāgrahaṃ neṣṭvevamevainānvyatyāsaṃ gṛhṇītaḥ

Now, when they take the Vasatîvarî water (into the havirdhâna shed) by the front door, the Neshtri takes in the Parisrut-liquor by the back door. From the south they bring in the drinking vessels. The Adhvaryu, seated in front of the axle, with his face towards the west, draws the cups of Soma; and the Neshtri, seated behind the axle, with his face towards the east, draws the cups of Surâ. The Adhvaryu draws a cup of Soma, the Neshtri a cup of Surâ; the Adhvaryu draws a cup of Soma, the Neshtri a cup of Surâ: in this way they draw them alternately.