Śatapatha Brāhmaṇa ("of a hundred paths") 8.6.2.6

8.6.2.6

tasya śiro gāyatryaḥ | tā yadgāyatryo bhavanti gāyatraṃ hiśirastisro bhavanti trivṛddhi śiraḥ pūrvārdha upadadhāti purastāddhīdaṃ śiraḥ

Gâyatrîs are his head; and as to its being Gâyatrîs, it is because the head is of Gâyatrî nature. There are three, for the head is threefold. He places them on the forepart (of the altar), for the head (of the animal or bird) is in front.