Śatapatha Brāhmaṇa ("of a hundred paths") 2.3.4.12

2.3.4.12

athaindrāgnī | ubhā vāmindrāgnī āhuvadhyā ubhā rādhasaḥ saha mādayadhyai ubhā dātārāviṣāṃ rayīṇāmubhā vājasya sātaye huve vāmityeṣa vā indro ya eṣa tapati sa yadastameti tadāhavanīyam praviśati tadubhāvevaitatsaha santā upatiṣṭhata ubhau me saha santau dattāmiti tasmādaindrāgnī

Thereupon the verse to Indra and Agni (Vâg. S. III, 13), 'You two, O Indra and Agni, I will invoke; you two I will delight together with kindly office; you two, the givers of strength and wealth,--you two I invoke for the obtainment of strength!' Indra, doubtless, is the same as that burning (sun); when he sets, then he enters the Âhavanîya;--hence he now approaches these two that are thus united, thinking, 'May the two, united, grant me favours:' this is why the Indra-Agni (verse is muttered).