Śatapatha Brāhmaṇa ("of a hundred paths") 8.3.1.13

8.3.1.13

yadveva diśyā upadadhāti | candāṃsi vai diśaḥ paśavo vai candāṃsyannam paśavo madhyam madhyamā citirmadhyatastadannaṃ dadhāti tā anantarhitāḥ svayamātṛṇāyā upadadhāti prāṇo vai svayamātṛṇānantarhitaṃ tatprāṇādannaṃ dadhātyuttarā uttaraṃ tatprāṇādannaṃ dadhāti retaḥsicorvelayā pṛṣṭayo vai retaḥsicau madhyamu pṛṣṭayo madhyata evāsminnetadannaṃ dadhāti sarvata upadadhāti sarvata evāsminnatadannaṃ dadhāti

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.'