Shatapatha Brahmana 10.4.1.23

10.4.1.23

athādhyātmam prāṇo vā arkastasyānnameva kyaṃ tadarkyaṃ yajuṣṭaḥ prāṇa eva māhāṃstasyānnameva vrataṃ tanmahāvrataṃ sāmataḥ prāṇa u evoktasyānnameva thaṃ tadukthamṛktastadetadekaṃ sattredhākhyāyate sa eṣa evaiṣo'dhidevatamayamadhyātmam

Now as to the body. The Arka (flame), doubtless, is the breath (vital air), and the Kya is its food: that 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. And, indeed, that (Agni) is that (sun) as to the deity, and this (breath) as to the body.