Śatapatha Brāhmaṇa ("of a hundred paths") 7.2.1.16

7.2.1.16

tisra iṣṭakā bhavanti | āsandī śikyaṃ rukbhapāśa iṇḍve tadaṣṭāvaṣṭākṣarā gāyatrī gāyatro 'gniryāvānagniryāvatyasya mātrā tāvataiva tatpāpmānaṃ nirṛtimapahate

There are three bricks, the seat, the netting, the sling of the gold plate, and the two pads; that makes eight;--the Gâyatrî consists of eight syllables, and Agni is Gâyatra: as great as Agni is, as great as is his measure, by so much he thus repels evil, corruption.