Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.2.9

7.3.2.9

āgneyīranvāha | agnirūpāṇāmupāptya kāmavatīḥ kāmānāmupāptyai gāyatrīrgāyatro 'gniryāvānagniryāvatyasya mātrā tāvataivainametadreto bhūtaṃ siñcati tisrasrivṛdagniryāvānagniryāvatyasya mātrā tāvataivainametadreto bhūtaṃ siñcati tāḥ sapta sampadyante saha triranūktābhyāṃ saptacitiko 'gniḥ sapta 'rtavaḥ saṃvatsaraḥ saṃvatsaro 'gniryāvānagniryāvatyasya mātrā tāvattadbhavatyupāṃśvanvāha reto vā atra yajña upāṃśu vai retaḥ sicyate paścādanubruvannanveti candobhirevaitadyajñam paścādabhirakṣanneti

Verses addressed to Agni he recites for the obtainment of Agni's forms;--and such as relate to desire, for the obtainment of his desires;--Gâyatrî ones,--Agni is Gâyatra: as great as Agni is, as great as is his measure, with so much he thus pours him forth as seed;--with three (verses),--Agni is threefold: as great as Agni is, as great as is his measure, with so much he thus pours him forth as seed. These (three), with the (first and last verses) recited thrice, amount to seven,--of seven layers consists the fire-altar, seven seasons are a year, and the year is Agni: as great as Agni is, as great as is his measure, so great does this become. He recites in a low voice, for here in the sacrifice seed is (cast), and seed is cast silently. He (the Hotri) marches reciting behind (the bricks carried by the attendants); he thus marches, defending the sacrifice by the metres from behind.