Śatapatha Brāhmaṇa ("of a hundred paths") 3.9.4.16

3.9.4.16

sa upasṛjati | śvātrā stha vṛtratura iti śivā hyāpastasmādāha śvātrā stheti vṛtratura iti vṛtraṃ hyetā aghnanrādhogūrtā amṛtasya patnīrityamṛtā hyāpastā devīrdevatremaṃ yajñaṃ nayateti nātra tirohitamivāstyupahūtāḥ somasya pibateti tadupahūtā eva prathamabhakṣaṃ somasya rājño bhakṣayanti

He pours it with (Vâg. S. VI, 34), 'Ye are grateful, the subduers of Vritra;'--the waters indeed are propitious: therefore he says, 'Ye are grateful;' and 'the subduers of Vritra' he says because they did slay Vritra;--'the beneficent wives of the immortal (Soma);' for the waters are immortal;--'Ye goddesses, lead this sacrifice to the gods!' there is nothing obscure in this; 'Invited, drink ye of Soma!' Thus invited they drink the first draught of king Soma.