Śatapatha Brāhmaṇa ("of a hundred paths") 1.8.2.10

1.8.2.10

sa vai khalu barhiḥ prathamaṃ yajati | tadvai kaniṣṭhaṃ candaḥ sadgāyatrī prathamā candasāṃ yujyate tadu tadvīryeṇaiva yacyeno bhūtvā divaḥ somamāharattadayathāyatham manyante yatkaniṣṭhaṃ candaḥ sadgāyatrī prathamā candasāṃ yujyate 'thātra yathāyathaṃ devāścandāṃsyakalpayannanuyājeṣu netpāpavasyasamasaditi

In the first place he makes offering to the Barhis (sacrificial-grass covering). Though the smallest metre, the gâyatrî is yoked first of the metres; and this on account of its strength, since, having become a falcon, it carried off the Soma from heaven. They consider it unseemly, however, that the gâyatrî, being the smallest metre, should be yoked first of the metres; and the gods accordingly arranged the metres here, at the after-offerings, so as it ought to be, 'lest there should be a confusion.'