Shatapatha Brahmana 10.4.1.22

10.4.1.22

sa eṣa evārko ya eṣa tapati tasyaitadannaṃ kyameṣa candramāstadarkyaṃ yajuṣṭa eṣa eva mahāṃstasyaitadannaṃ vrataṃ tanmahāvrataṃ sāmata eṣa u evoktasyaitadannaṃ thaṃ tadukthamṛktastadetadekaṃ sattredhākhyāyata ityadhidevatam

He who shines yonder is indeed that Arka (flame), and that moon is his food, the Kya: 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. Thus much as to the deity.