Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.3.8

1.4.3.8

agniṃ dūtaṃ vṛṇīmaha iti | ya evāyam madhyamaḥ prāṇaetamevaitayā samindhe sā haiṣāntasthā prāṇānāmato hyanya ūrdhvāḥprāṇā ato 'nye 'vāñco 'ntasthā ha bhavatyantastāmenam manyante ya evametāmantasthām prāṇānāṃ veda

Further, 'Agni we choose for messenger,'--what central breath there is (in the body), that he kindles with this (the eighth verse): that one indeed is the internal motive force of the breathings; from it (two) others tend upwards, and from it (two) others tend downwards, for it is indeed the internal motive force. And whosoever knows that internal motive force of the breathings, him they regard as the internal motive force.