Shatapatha Brahmana 8.3.1.3

8.3.1.3

ta indrāgnī abruvan | yuvaṃ na imāṃ tṛtīyāṃ citimupadhattamiti kiṃ nau tato bhaviṣyatīti yuvameva naḥ śreṣṭhau bhaviṣyatha iti tatheti tebhya etāmindrāgnī tṛtīyāṃ citimupādhattāṃ tasmādāhurindrāgnī eva devānāṃ śreṣṭhāviti

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.'