Śatapatha Brāhmaṇa ("of a hundred paths") 11.2.5.1

11.2.5.1

api ha vā etarhi devebhyo'śvamedhamālabhante tadāhuḥ prākṛto'śvamedha itītara innūnaṃ sa tadvā eṣa evāśvamedho yaccandramāḥ

And, verily, even on this occasion, they slaughter the sacrificial horse (Asvamedha) as a sacrifice to the gods: of this (New and Full-moon sacrifice) they say, 'It is the original (normal) Asvamedha;' and that (real Asvamedha), indeed, is just the other (modified one); for, indeed, the Asvamedha is the same as the moon.