Śatapatha Brāhmaṇa ("of a hundred paths") 2.3.4.32

2.3.4.32

yadvā āhavanīyamupatiṣṭhate | paśūṃstadyācate tasmāttamuccāvacaiścandobhirupatiṣṭhata uccāvacā iva hi paśavo 'tha yadgārhapatyam puruṣāṃstadyācate tadgāyatram prathamaṃ tricaṃ gāyatraṃ vā agneścandaḥ svenaivainametaccandasopaparaiti

Now when he approaches the Âhavanîya, he prays for cattle: he therefore approaches it with metres great and small, since cattle are of great and small size. And when he approaches the Gârhapatya, he prays for men: hence the first tristich is in the gâyatrî metre, since the gâyatrî is Agni's metre, and he thus approaches him with his own metre.