Śatapatha Brāhmaṇa ("of a hundred paths") 6.1.3.19

6.1.3.19

yadvevāṣṭāvagnirūpāṇi | aṣṭākṣarā gāyatrī tasmādāhurgāyatro 'gniriti so 'yaṃ kumāro rūpāṇyanuprāviśanna vā agniṃ kumāramiva paśyantyetānyevāsya rūpāṇi paśyantyetāni hi rūpāṇyanuprāviśat

And because there are eight forms of Agni-- the Gâyatrî consisting of eight syllables--therefore they say, 'Agni is Gâyatra.' That boy entered into the forms one after another; for one never sees him as a mere boy (kumâra), but one sees those forms of his, for he assumed those forms one after another.