Brihadaranyaka Upanishad 1.2.7.p1

1.2.7.p1

iti prathamādhyāye dvitīyaṃ brāhmaṇam

He desired, 'Let this body of mine be fit for a sacrifice, and let me be embodied through this,' (and entered it). Because that body swelled (Ashwat), therefore it came to be called Ashwa (horse). And because it became fit for a sacrifice, therefore the horse sacrifice came to be known as Ashwamedha. He who knows it thus indeed knows the horse sacrifice. (Imagining himself as the horse and) letting it remain free, he reflected (on it). After a year he sacrificed it to himself, and dispatched the (other) animals to the gods. Therefore (priests to this day) sacrifice to Prajâpati the sanctified (horse) that is dedicated to all the gods. He who shines yonder is the horse sacrifice; his body is the year. This fire is Arka; its limbs are these worlds. So these two (fire and the sun) are Arka and the horse sacrifice. These two again become the same god, Death. He (who knows this) conquers further death, death cannot overtake him, it becomes his self, and he becomes one with these deities.

to this day similarly sacrifice to Prajâpati the sanctified horse that is dedicated to all the gods.