Śatapatha Brāhmaṇa ("of a hundred paths") 8.3.1.1

8.3.1.1

tṛtīyāṃ citimupadadhāti | etadvai devā dvitīyāṃ citiṃ citvā samārohanyadūrdhvam pṛthivyā arvācīnamantarikṣāttadeva tatsaṃskṛtya samārohan

They said to Indra and Agni, 'Lay ye down for us this third layer!'--'What will accrue unto us therefrom? Ye two shall be the best of us!'--'So be it!' Accordingly Indra and Agni laid down for them that third layer; and hence people say, 'Indra and Agni are the best of gods.'