Śatapatha Brāhmaṇa ("of a hundred paths") 5.2.3.8

5.2.3.8

atha śvo bhūte | aindrāgnaṃ dvādaśakapālam puroḍāśaṃ nirvapati tena yatheṣṭyaivaṃ yajate yatra vā indro vṛtramahaṃstadasya bhītasyendriyaṃ vīryamapacakrāma sa etena haviṣendriyaṃ vīryam punarātmannadhatta tatho evaiṣa etena haviṣendriyaṃ vīryamātmandhatte tejo vā agnirindriyaṃ vīryamindra ubhe vīrye parigṛhya sūyā iti tasmādaindrāgno dvādaśakapālaḥ puroḍāśo bhavati tasyarṣabho 'naḍvāndakṣiṇā sa hi vahenāgneya āṇḍābhyāmaindrastasmādṛṣabho 'naḍvāndakṣiṇā

And on the following day he prepares a cake on twelve potsherds for Indra and Agni, and offers it in the same way as an (ordinary) ishti. Now when Indra slew Vritra, that vigour and energy of his went out of him, being frightened: by this offering he again possessed himself of that vigour and energy. And in like manner does this (Sacrificer) by this offering possess himself of vigour and energy; for Agni is fiery spirit, and Indra is vigour and energy: 'May I be consecrated, having embraced both these energies!' thus he thinks: hence there is a cake on twelve potsherds for Indra and Agni. A bull is the fee for this (offering), for by his shoulder he is of Agni's nature, and by his testicles he is of Indra's nature: therefore a bull is the fee for it.