Śatapatha Brāhmaṇa ("of a hundred paths") 3.5.1.10

3.5.1.10

atha yaccaturviṃśativikramā purastādbhavati | caturviṃśatyakṣarā vai gāyatrī pūrvārdho vai yajñasya gāyatrī pūrvārdhe eṣa yajñasya tasmāccaturviṃśativikramā purastādbhavatyeṣā mātrā vedeḥ

And as to (the altar) being twenty-four steps broad in front;--the Gâyatrî consists of twenty-four syllables, and the Gâyatrî is the fore-part of the sacrifice: this is why it is twenty-four steps broad in front. This is the measure of the altar.