Śatapatha Brāhmaṇa ("of a hundred paths") 8.1.3.8

8.1.3.8

sa vai purastādupadhāya paścādupadadhāti | prāṇo hāpāno bhūtvāṅgulyagrebhya iti saṃcaratyapāna u ha prāṇo bhūtvāṅgulyagrebhya iti saṃcarati tadyatpurastādupadhāya paścādupadadhātyenāvevaitatprāṇau saṃtanoti saṃdadhāti tasmādetau prāṇau saṃtatau saṃhitau

Now, when he has laid down (the bricks) in front, he lays down those at the back (of the altar); for the upward air, becoming the downward air, passes along thus from the tips of the fingers; and the downward air, becoming the upward air, passes along thus from the tips of the toes: hence when, after laying down (the bricks) in front, he lays down those at the back, he thereby makes these two breathings continuous and connects them; whence these two breathings are continuous and connected.