Shatapatha Brahmana 10.4.1.4

10.4.1.4

sa eṣa evārkaḥ yametamatrāgnimāharanti tasyaitadannaṃkyaṃ yo'yamagniścitastadarkyaṃ yajuṣṭa eṣa eva mahāṃstasyaitadannaṃ vrataṃ tanmahāvrataṃ sāmata eṣa u evoktasyaitadannaṃ thaṃ tadukthamṛktastadetadekaṃ satredhākhyāyate

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