Śatapatha Brāhmaṇa ("of a hundred paths") 4.1.4.4

4.1.4.4

sa kṣatraṃ varuṇaḥ | brahma mitramupamantrayāṃ cakra upa māvartasva saṃsṛjāvahai purastvā karavai tvatprasūtaḥ karma karavā iti tatheti tau samasṛjetāṃ tata eṣa maitrāvaruṇo graho 'bhavat

Varuna, the nobility, then called upon Mitra, the priesthood, saying, 'Turn thou unto me that we may unite: I will place thee foremost, sped by thee, I will do deeds!'--'So be it!' So the two united; and therefrom resulted that graha to Mitra and Varuna.