Shatapatha Brahmana 1.3.5.15

1.3.5.15

sa yadyetannodāśaṃseta | apyekaikāmevānavānannanubrūyāttadekaikayaivemāṃllokāṃtsaṃtanotyekaikayemāṃllo kāṃtspṛṇute 'tha yatprāṇaṃ dadhāti gāyatrī vai prāṇaḥ sa yatkṛtsnāṃ gāyatrīmanvāha tatkṛtsnaṃ prāṇaṃ dadhāti tasmādekaikāmevānavānannanubrūyāt

If, however, he do not care to undertake this, he may also recite one (verse) at a time without drawing breath: he thereby spreads those worlds one by one, gains those worlds one by one. The reason why he takes breath, is that the gâyatrî is indeed breath; and that by reciting a complete gâyatrî verse, he accordingly bestows complete breathing (on the sacrificer): let him therefore recite one (verse) at a time without breathing.