Śatapatha Brāhmaṇa ("of a hundred paths") 7.4.1.18

7.4.1.18

uttānamupadadhāti | etadvai devā abruvanyadi vā imāvarvāñcā upadhāsyāmaḥ sarvamevedam pradhakṣyato yadyu parāñcau parāñcāveva tapsyato yadyu samyañcāvantaraivaitāvetajjyotirbhaviṣyatyatho anyo 'nyaṃ hiṃsiṣyata iti te 'rvāñcamanyamupādadhuḥ parāñcamanyaṃ sa eṣa raśmibhirarvāṅ tapati rukbhaḥ prāṇaireṣa ūrdhvaḥ puruṣaḥ prāñcamupadadhāti prāṅ hyeṣo 'gniścīyate

He lays him down on his back;--for the gods at that time said, 'If we lay down these two both looking hitherwards, they will burn up everything here; and if (we lay) both so as to be turned away from here, they will give warmth only in the opposite direction; and if facing each other, then there will be light only between those two, and they will injure each other.' They laid down the one so as to look hitherwards, and the other so as to look away from here: that one (the sun), the gold disk, looking downwards, gives warmth by his rays, and that man (tends) upward by his vital airs. He lays him down (with the head) towards the east, for (with the head) towards the east this Agni (the fire-altar) is built up.