Shatapatha Brahmana 10.4.1.21

10.4.1.21

sa eṣa evārkaḥ yo'yamagniścitastasyaitadannaṃ kyameṣa saumyo'dhvarastadarkyaṃ yajuṣṭa eṣa eva mahāṃstasyaitadannaṃ vrataṃ tanmahāvrataṃ sāmata eṣa u evoktasyaitadannaṃ thaṃ tadukthamṛktastadetadekaṃ sattredhākhyāyate sa etenānnena sahordhva udakrāmatsa yaḥ sa udakrāmadasau sa ādityo'tha yena tenānnena sahodakrāmadeṣa sa candramāḥ

Now that Arka (flame) is this very fire-altar built here; and the Kya is this his food, to wit, the Soma-sacrifice: 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. With this food he went upwards; and he who went upwards is yonder sun, and that food wherewith he went up is that moon.