Shatapatha Brahmana 8.3.1.9

8.3.1.9

viśvakarmā tvā sādayatviti | viśvakarmā hyetāṃ tṛtīyāṃ citimapaśyadantarikṣasya pṛṣṭhe vyacasvatīm prathasva tīmityantarikṣasya hyetatpṛṣṭhaṃ vyacasvatprathasvadantarikṣaṃ yacāntarikṣaṃ dṛṃhāntarikṣam mā hiṃsīrityātmānaṃ yacātmānaṃ dṛṃhātmānam mā hiṃsīrityetat

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.'