Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.1.2

1.1.2

aharvā aśvaṃ purastānmahimānvajāyata tasya pūrve samudre yoniḥ rātrirenaṃ paścānmahimānvajāyata tasyāpare samudre yoniretau vā aśvaṃ mahimānāvabhitaḥ saṃbabhūvatuḥ | hayo bhūtvā devānavahadvājī gandharvānarvāsurānaśvo manuṣyānsamudra evāsya bandhuḥ samudro yoniḥ

Verily Day arose after the horse as the (golden) vessel, called Mahiman (greatness), which (at the sacrifice) is placed before the horse. Its place is in the Eastern sea. The Night arose after the horse as the (silver) vessel, called Mahiman, which (at the sacrifice) is placed behind the horse. Its place is in the Western sea. Verily, these two vessels (or greatnesses) arose to be on each side of the horse.
As a racer he carried the Devas, as a stallion the Gandharvas, as a runner the Asuras, as a horse men. The sea is its kin, the sea is its birthplace.

1.1.2.p1

iti prathamādhyāye prathamaṃ brāhmaṇam