Śatapatha Brāhmaṇa ("of a hundred paths") 1.8.2.13

1.8.2.13

athāgniruttamaḥ | gāyatrī vā agnistadgāyatrīmuttamāmakurvannevaṃ yathāyathena kLptena candāṃsi pratyatiṣṭhaṃstasmādidamapāpavasyasam

Then Agni is the last (to whom offering is made). Agni assuredly is the gâyatrî; and therefore they placed the gâyatrî last. In this way they established the metres in complete and proper order; and hence no confusion here takes place.