Shatapatha Brahmana 10.3.5.5

10.3.5.5

ayamevākāśo jūḥ yo'yamantarātmannākāśa etaṃ hyākāśamanu javate tadetadyajuḥ prāṇaścākāśaśca yacca jūśca tasmādyajuḥ prāṇa eva yatprāṇo hyeti

And this course (gûh) is space--this space which is inside the body--for along this space it (the breath) courses; and the Yagus is both the breath and space,--the 'yat' and the 'gûh': hence 'yagus.' And the 'yat' (moving) is the breath, for the breath moves.