Śatapatha Brāhmaṇa ("of a hundred paths") 13.4.2.16

13.4.2.16

sa āha devā āśāpālāḥ etaṃ devebhyo'śvam medhāya prokṣitaṃ rakṣatetyuktā mānuṣā āśāpālā athaite daivā āpyāḥ sādhyā anvādhyā marutastameta ubhaye devamanuṣyāḥ saṃvidānā apratyāvartayantaḥ saṃvatsaraṃ rakṣanti tadyaṃ na pratyāvartayantyeṣa vā eṣa tapati ka u hyetamarhati pratyā !vayituṃ yadyenam pratyāvartayeyuḥ parāgevedaṃ sarvaṃ syāttasmādapratyāvartayanto rakṣanti

He says, 'O ye gods, guardians of the regions, guard ye this horse, consecrated for offering unto the gods!' The (four kinds of) human guardians of the (four) regions have been told, and these now are the divine ones, to wit, the Âpyas, Sâdhyas, Anvâdhyas and Maruts; and both of these, gods and men, of one mind, guard it for a year without turning (driving) it back. The reason why they do not turn it back, is that it is he that shines yonder,--and who, forsooth, is able to turn him back? But were they to turn it back, everything here assuredly would go backward (go to ruin): therefore they guard it without turning it back.