Shatapatha Brahmana 10.4.1.15

10.4.1.15

sa eṣa evārkaḥ yo'yamagniścitastasyaitadannaṃ kyameṣa mahāvratīyo grahastadarkyaṃ yajuṣṭa eṣa eva mahāṃstasyaitadannaṃ vrataṃ tanmahāvrataṃ sāmata eṣa u evoktadannaṃ thaṃ tadukthamṛktastadetadekaṃ sattredhākhyāyate

Now that Arka (flame) is this very fire-altar built here; and the Kya is this his food, to wit, the Mahâvratîya-graha: that (combined) makes the Arkya in respect of the Yagus. And the Great one (mahân) is this (Agni), and this rite (vrata) is his food: that makes the Mahâvrata in respect of the Sâman. And 'uk' is this (Agni), and 'tha' his food: that makes the (Mahad) Uktha in respect of the Rik. Thus, whilst being only one, this is accounted threefold.