Śatapatha Brāhmaṇa ("of a hundred paths") 4.1.3.1

4.1.3.1

vāggha vā asyaindravāyavaḥ | etannvadhyātmamindro ha yatra vṛtrāya vajram prajahāra so 'balīyānmanyamāno nāstṛṣītīva bibhyannilayāṃ cakre tadevāpi devā apanyalayanta

The Aindra-vâyava (graha), forsooth, is his speech; and as such belonging to his self. Now Indra, when he had hurled the thunderbolt at Vritra, thinking himself to be the weaker, and fearing lest he had not laid him low, hid himself. The gods also hid themselves away in the same place.