Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.4.21

4.2.4.21

vatso vā eṣaḥ | etasyai gāyatryai yajño vai gāyatrī dvādaśa stotrāṇi dvādaśa śastrāṇi taccaturviṃśatiścaturviṃśatyakṣarā vai gāyatrī tasyā eṣa vatsastaṃ yadgopāyanti gopāyanti vā imānvatsāndohāya yadidam payo duhra evamiyaṃ gāyatrī yajamānāya sarvānkāmāndohātā iti tasmādvai gopāyanti

Moreover, this (graha) is Gâyatrî's calf, Gâyatrî being the sacrifice,--there are twelve chants and twelve recitations: that makes twenty-four, and of twenty-four syllables consists the Gâyatrî. This is her calf;--when they watch it, then they watch these calves for the sake of the milking: 'as they yield this milk, even so may this Gâyatrî yield all the sacrificer's wishes,'--this is why they watch it.