Śatapatha Brāhmaṇa ("of a hundred paths") 10.4.1.5

10.4.1.5

athendrāgnī vā asṛjyetām brahma ca kṣatraṃ cāgnireva brahmendraḥ kṣatraṃ tau sṛṣṭau nānaivāstāṃ tāvabrūtāṃ na vā itthaṃ santau śakṣyāvaḥ prajāḥ prajanayitumekaṃ rūpamubhāvasāveti tāvekaṃ rūpamubhāvabhavatām

Now Indra and Agni were created as the Brahman (priesthood) and the Kshatra (nobility): the Brahman was Agni and the Kshatra Indra. When created, the two were separate. They spake, 'Whilst being thus, we shall be unable to produce creatures (people): let us both become one form!' The two became one form.