Śatapatha Brāhmaṇa ("of a hundred paths") 6.4.2.5

6.4.2.5

gāyatrībhiḥ | prāṇo gāyatrī prāṇamevāsminnetaddadhāti nnetaddadhāti tāsāṃ nava padāni nava vai prāṇāḥ saptatisṛbhistrayo vai prāṇāḥ prāṇa udāno vyānastānevāsmi śīrṣannavāñco dvau tānevāsminnetaddadhāti

With Gâyatrî verses (he performs),--the Gâyatrî is the vital air: he thus lays vital air into him. With three (verses);--there are three vital airs, the out-breathing, the in-breathing, and the through-breathing: these he thus lays into him. These (verses) consist of nine feet, for there are nine vital airs, seven in the head, and two downward ones: these he thus lays into him.