Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.1.30

1.4.1.30

aśvo na devavāhana iti | aśvo ha vā eṣa bhūtvā devebhyo yajñaṃ vahati yadvai netyṛcyomiti tattasmādāhāśvo na devavāhana iti

'Yea, as a horse that bears the gods,' for having become a horse he does indeed carry the sacrifice to the gods: the (word) 'na' which occurs in this verse has the meaning of 'om' (verily); hence he says, 'Yea, as a horse that bears to the gods.'