Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.2.7

1.2.7

so'kāmayata medhyaṃ ma idam̐ syādātmanvyanena syāmiti | tato'śvaḥ samabhavadyadaśvattanmedhyamabhūditi tadevāśvamedhasyāśvamedhatvam | eṣa ha vā aśvamedhaṃ veda ya enamevaṃ veda | tamanavarudhyaivāmanyata tam̐ saṃvatsarasya parastādātmana ālabhata | paśūndevatābhyaḥ pratyauhat | tasmātsarvadevatyaṃ prokṣitaṃ prājāpatyamālabhante | eṣa ha vā aśvamedho ya eṣa tapati tasya saṃvatsara ātmāyamagnirarkastasyeme lokā ātmānastāvetāvarkāśvamedhau so punarekaiva devatā bhavati mṛtyurevāpa punarmṛtyuṃ jayati nainaṃ mṛtyurāpnoti mṛtyurasyātmā bhavatyetāsāṃ devatānāmeko bhavati

He desired that this body should be fit for sacrifice (medhya), and that he should be embodied by it. Then he became a horse (asva), because it swelled (asvat), and was fit for sacrifice (medhya); and this is why the horse-sacrifice is called Asva-medha.
Verily he who knows him thus, knows the Asvamedha. Then, letting the horse free, he thought, and at the end of a year he offered it up for himself, while he gave up the (other) animals to the deities. Therefore the sacrificers offered up the purified horse belonging to Pragâpati, (as dedicated) to all the deities.
Verily the shining sun is the Asvamedha-sacrifice, and his body is the year; Agni is the sacrificial fire (arka), and these worlds are his bodies. These two are the sacrificial fire and the Asvamedha-sacrifice, and they are again one deity, viz. Death. He (who knows this) overcomes another death, death does not reach him, death is his Self, he becomes one of those deities.

1.2.7.p1

iti prathamādhyāye dvitīyaṃ brāhmaṇam