Śatapatha Brāhmaṇa ("of a hundred paths") 4.1.1.2

4.1.1.2

atha yasmādupāṃśurnāma | aṃśurvai nāma grahaḥ sa prajāpatistasyaiṣa prāṇastadyadasyaiṣa prāṇastasmādupāṃśurnāma

Now as to why it is called Upâmsu. There is a graha called Amsu, that is Pragâpati: his out-breathing is this (graha); and because it is his out-breathing, therefore it is called Upâmsu.