Śatapatha Brāhmaṇa ("of a hundred paths") 4.4.5.17

4.4.5.17

athājyasyopastīrya | puroḍāśamavadadhadāhāgnīvaruṇābhyāmanubrūhīti tatsviṣṭakṛte sa yannāgnaya ityāha nedagni varuṇo gṛhṇāditi sa yadyamutrarjīṣasya dviravadyedathātra sakṛdyadyu na nādriyetāthopariṣṭāddvirājyasyābhighārayatyāśrāvyāhāgnīvaruṇau yajeti vaṣaṭkṛte juhoti

Then, having made an underlayer of ghee, he says, while putting the (remainder of the) cake (into the spoon), 'Recite the invitatory prayer to Agni and Varuna!' This is for (Agni) Svishtakrit; and as to why he does not say 'To Agni,' it is lest Varuna might seize upon Agni. If before he has cut twice from the Soma-husks, he now does so once, but if (he did) not, he need not heed it. He then bastes it twice with butter on the upper side; and having called for the Sraushat, he says, 'Recite the offering prayer to Agni and Varuna!' and offers as the Vashat is uttered.