Shatapatha Brahmana 11.1.6.18

11.1.6.18

sa prajāpatirindram putramabravīt anena tvā kāmapreṇa yajñena yājayāni yena māmidam parameṣṭhyayīyajaditi tatheti tamayājayatsa iṣṭvākāmayatāhamevedaṃ sarvaṃ syāmiti sa vāgabhavadvāgvā idaṃ sarvaṃ tasmādāhurindro vāgiti

Pragâpati spake unto his son Indra, 'Let me perform for thee this wish-fulfilling sacrifice which Parameshthin has just performed for me.'--'So be it!' he said. He accordingly performed it for him. Having sacrificed, he (Indra) desired, 'Would that I were everything here!' He became speech (vâk), for speech is everything here; whence they say, 'Indra is Vâk.'