Śatapatha Brāhmaṇa ("of a hundred paths") 13.7.1.3

13.7.1.3

tasyāgniṣṭudagniṣṭomaḥ prathamamaharbhavati agnirvā agniṣṭudagniṣṭomo'gnimukhā u vai sarve devāḥ sarveṣāṃ devānāmāptyai tasyāgneyā grahā bhavantyagniyyaḥ purorucaḥ sarvamāgneyamasaditi

The first day thereof is an Agnishtut Agnishtoma, for the sake of his gaining and winning all the gods, for the Agnishtut Agnishtoma is Agni; and all the gods have Agni (the sacrificial fire) for their mouth, The cups of Soma thereat pertain to Agni, and so do the Puroruk formulas pertain to Agni, in order that everything should pertain to Agni.