Shatapatha Brahmana 8.1.3.9

8.1.3.9

atha dakṣiṇata upadhāyottarata upadadhāti | vyāno hodāno bhūtvāṅgulyagrebhya iti saṃcaratyudāna u ha vyāno bhūtvāṅgulyagrebhya iti saṃcarati tadyaddakṣiṇata upadhāyottarata upadadhātyetāvevaitatprāṇau saṃtanoti saṃdadhāti tasmādetau prāṇau saṃtatau saṃhitau

And when he has laid down those on the right side, he lays down those on the left side; for the outward air, becoming the circulating air, passes along thus from the tips of the fingers; and the circulating air, becoming the outward air, passes along thus from the tips of the fingers: hence when, after laying down (the bricks) on the right side, he lays down those on the left side, he thereby makes these two breathings continuous and connects them; whence these two breathings are continuous and connected.