Śatapatha Brāhmaṇa ("of a hundred paths") 14.2.2.40

14.2.2.40

atha mahāvīrādupayamanyām pratyānayati svāhā saṃ jyotiṣā jyotiriti jyotirvā itarasminpayo bhavati jyotiritarasyāṃ te hyetadubhe jyotiṣī saṅgacete avaraṃ svāhākāraṃ karoti parāṃ devatāmasāveva banduḥ

He then pours (the remaining milk and ghee) from the Mahâvîra into the supporting-tray with, 'Hail, light with light!'--for light indeed the milk was in the one (vessel), and light it is in the other, and these two lights thus unite with each other. The call of 'hail!' he places first, and the deity last: the mystic import of this is the same as before.