Śatapatha Brāhmaṇa ("of a hundred paths") 5.3.2.6

5.3.2.6

athātacya dadhi | vināṭa āsicya rathaṃ yuktvābadhya dedīyitavā āha tadyatsvayamuditaṃ navanītaṃ tadājyam bhavati varuṇyaṃ vā etadyanmathitamathaitanmaitraṃ yatsvayamuditaṃ tasmātsvayamuditamājyam bhavati

Thereupon having curdled the (milk into) curds, and poured it into a leathern bag; and having put (the horses) to the cart, and fastened (the bag on the cart), he tells it to 'fly away.' This is that (kind of) fresh butter which is self-produced; for that which is churned belongs to Varuna, and that which is self-produced belongs to Mitra: therefore it is self-produced butter.