Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.26

3.2.1.26

indro ha vā īkṣāṃ cakre | mahadvā ito 'bhvaṃ janiṣyate yajñasya ca mithunādvācaśca yanmā tannābhibhavediti sa indra eva garbho bhūtvaitanmithunam praviveśa

Indra then thought within himself, 'Surely a great monster will spring from this union of Yagña and Vâk: [I must take care] lest it should get the better of me.' Indra himself then became an embryo and entered into that union.