Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.1.16

1.1.1.16

yadvevāpaḥ praṇayati | devānha vai yajñena yajamānāṃstānasurarakṣasāni rarakṣurna yakṣyadhva iti tadyadarakṣaṃstasmādrakṣāṃsi

Another reason why he brings forward water is this: whilst the gods were engaged in performing sacrifice, the Asuras and Rakshas forbade (raksh) them, saying, 'Ye shall not sacrifice!' and because they forbade (raksh), they are called Rakshas.