Śatapatha Brāhmaṇa ("of a hundred paths") 2.1.4.14

2.1.4.14

sa vai bhūrbhuva iti | etāvataiva gārhapatyamādadhātyatha yatsarvairādadhyātkenāhavanīyamādadhyāddve akṣare pariśinaṣṭi teno etānyayātayāmāni bhavanti taiḥ sarvaiḥ pañcabhirāhavanīyamādadhāti bhūrbhuvaḥ svariti tānyaṣṭāvakṣarāṇi sampadyante 'ṣṭākṣarā vai gāyatrī gāyatramagneścandaḥ svenaivainametaccandasādhatte

'Bhûr bhuvah!' this much he utters while laying down the Gârhapatya fire; for if he were to lay it down with all (three words), wherewith should he lay down the Âhavanîya? Two syllables he leaves over, and thereby those (five syllables) become of renewed efficacy; and with all the five syllables--'Bhûr bhuvah svah'--he lays down the Âhavanîya. Thus result eight syllables; for of eight syllables consists the gâyatrî, and the gâyatrî is Agni's metre: he thus establishes that (fire) by means of its own metre.