Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.3.7

1.1.3.7

tāḥ savye pāṇau kṛtvā | dakṣiṇenodiṅgayatyupastautyevainā etanmahayatyeva devīrāpo agreguvo agrepuva iti devyo hyāpastasmādāha devīrāpa ityagreguva iti tā yatsamudraṃ gacanti tenāgreguvo 'grepuva iti tā yatprathamāḥ somasya rājño bhakṣayanti tenāgrepuvo 'gra imamadya yajñaṃ nayatāgre yajñapatiṃ sudhātuṃ yajñapatiṃ devayuvamiti sādhu yajñaṃ sādhu yajamānamityevaitadāha

Having taken it (the water with the ladle) in his left hand, he makes it spirt upwards with his right hand, and eulogises and glorifies it, with the text (Vâg. S. I, 12 c): 'Shining (or divine) waters! ye the first-going, the first-drinking ones!' For the waters are shining; and for this reason he says, 'Shining waters!' 'First-going,' he calls them, because they flow towards the sea and are therefore going in front (or forwards). 'First-drinking,' he calls them, because they are the first that drink of king Soma and are therefore 'drinking first.' And further: 'Forward now lead this sacrifice, forward the Lord of Sacrifice, the liberal, god-loving Lord of Sacrifice!' whereby he says, 'Well (lead) the sacrifice, well the sacrificer!'