Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.1.17

2.2.1.17

aṣṭākapālāḥ sarve puroḍāśā bhavanti | aṣṭākṣarā vai gāyatrī gāyatramagneścandaḥ svenaivainametaccandasādhatte tāni sarvāṇi caturviṃśatiḥ kapālāni sampadyante caturviṃśatyakṣarā vai gāyatrī gāyatramagneścandaḥ svenaivainametaccandasādhatte

All these sacrificial cakes (for Agni) are on eight potsherds; for of eight syllables consists the (pâda of the) gâyatrî, and the gâyatrî is Agni's metre: with its own metre he accordingly establishes that fire. In all, these potsherds amount to twenty-four; for of twenty-four syllables consists the gâyatrî (stanza), and the gâyatrî is Agni's metre: with its own metre he accordingly establishes that fire.