Śatapatha Brāhmaṇa ("of a hundred paths") 11.1.3.2

11.1.3.2

sa yatpaurṇamāseneṣṭvā indrāya vimṛdhe'nunirvapatīndro vai yajñasya devatāthaidagnīṣomīyam paurṇamāsaṃ havirbhavati tatra nendrāya tveti kiṃ cana kriyata eteno hāsyaitatsendraṃ havirbhavatyetena sendro yajño'tha yadvimṛdhe tveti sarvā u hi mṛdho nāṣṭrāḥ paurṇamāsena hanti

And as to why, after performing the Full-moon sacrifice, he prepares (a cake) for Indra Vimridh, it is because Indra is the deity of the sacrifice; but the chief oblation of the Full-moon sacrifice belongs to Agni and Soma, and nothing is offered there with the formula 'To Indra (I offer) thee!' Hereby then that oblation comes to be shared by Indra, and so does the sacrifice come to be shared by Indra. And as to why (he offers) with 'To (Indra) Vimridh!' it is that by the Full-moon sacrifice he slays all scorners (mridh), all evil spirits.