Śatapatha Brāhmaṇa ("of a hundred paths") 1.7.2.25

1.7.2.25

sa yo devānāmāsīt | sa yavāyuvata hi tena devā yo 'surāṇāṃ so 'yavā na hi tenāsurā ayuvata

That (half-moon) which belonged to the gods is (called) yavan, for the gods possessed themselves (yu, 'to join') of it; and that which belonged to the Asuras is ayavan, because the Asuras did not possess themselves of it.