Śatapatha Brāhmaṇa ("of a hundred paths") 8.7.3.12

8.7.3.12

prothadaśvo na yavase | 'viṣyanyadā mahaḥ saṃvaraṇādvyasthāt ādasya vāto anuvāti śociradha sma te vrajanaṃ kṛṣṇamastīti yadā vā etasya vāto 'nuvāti śocirathaitasya vrajanaṃ kṛṣṇam bhavati triṣṭubhopadadhāti traiṣṭubho hi vāyurāgneyyāgnikarma hyaniruktayānirukto hi vāyuratha yadvāta ityāha vāto hi vāyuḥ

[He lays it down, with, Vâg. S. XV, 62; Rig-veda VII, 3, 2], 'When, like a snorting steed, that longeth for the pasture, he started forth from the great enclosure, then the wind fanned his flame, and black then was thy path;'--for when the wind fans his (Agni's) flame, then his path does become black. With a trishtubh verse he lays it down, because Vâyu (the wind) is of trishtubh nature; with one relating to Agni, because it is Agni's performance; with an undefined one, because Vâyu is undefined. And as to his saying 'the wind,' Vâyu indeed is the wind.