Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.3.22

1.6.3.22

atha yadindre sarve devāstasthānāḥ | tasmādāhurindraḥ sarvā devatā indraśreṣṭhā devā ityetaddha vai devāstredhaikadevatyā abhavantsa yo haivametadvedaikadhā haiva svānāṃ śreṣṭho bhavati

And further, since all the gods were abiding in Indra, for that reason they say that Indra is all the deities, that the gods have Indra for their chief (sreshtha). Thus the gods came in a three-fold way to consist of one deity; and he who knows this becomes individually the chief of his own (people).