Śatapatha Brāhmaṇa ("of a hundred paths") 8.1.2.1

8.1.2.1

atha paścāt | ayam paścādviśvavyacā ityasau vā ādityo viśvavyacā yadā hyevaiṣa udetyathedaṃ sarvaṃ vyaco bhavati tadyattamāha paścāditi tasmādetam pratyañcameva yantam paśyanti cakṣurhādityo bhūtvā paścāttasthau tadeva tadrūpamupadadhāti

And at the back (western part of the altar), with (Vâg. S. XIII, 56), 'This one behind, the all-embracer;'--the all-embracer, doubtless, is yonder sun, for as soon as he rises, all this embracing space comes into existence. And because he speaks of him as (being) 'behind,' therefore one sees him only when he goes towards the back (west). The Sun, indeed, having become the eye, remained behind: it is that form he now bestows (on Agni).