Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.1.22

6.2.1.22

yadveva caturviṃśatiḥ | caturviṃśatyakṣarā vai gāyatrī gāyatro 'gniryāvānagniryāvatyasya mātrā tāvataivainametatsaminddhe

And, again, why there are twenty-four,--the Gâyatrî consists of twenty-four syllables, and Agni is Gâyatra: as great as Agni is, as great as is his measure, by so much he thus kindles him.