Śatapatha Brāhmaṇa ("of a hundred paths") 4.1.2.11

4.1.2.11

prāṇodānau ha vā asyaitau grahau | tayorudite 'nyataraṃ juhotyanudite 'nyataram prāṇodānayorvyākṛtyai prāṇodānāvevaitadvyākaroti tasmādetau samānāveva santau nānevācakṣate prāṇa iti codāna iti ca

Now these two grahas are his out-breathing and in-breathing; one of them he offers after sunrise and the other before sunrise, in order to keep the out-breathing and in-breathing distinct from each other. He thus keeps the out-breathing and in-breathing distinct from each other: hence these two, even while being one and the same, are yet called differently 'out-breathing' and 'in-breathing.'