Śatapatha Brāhmaṇa ("of a hundred paths") 2.1.4.15

2.1.4.15

devānha vā agnī 'ādhāsyamānān | tānasurarakṣasāni rarakṣurnāgnirjaniṣyate nāgnī ādhāsyadhva iti tadyadarakṣaṃstasmādrakṣāṃsi

Now when the gods were about to set up their fires, the Asuras and Rakshas forbade them, saying, 'The fire shall not be produced; ye shall not set up your fires!' and because they thus forbade (raksh) them, they are called Rakshas.