Shatapatha Brahmana 11.2.5

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.

As to this, they say, 'For each foot of the sacrificial horse they offer an oblation;'--when he performs the Agnihotra in the evening and morning, he offers two oblations in the evening, and two in the morning--that makes four oblations: thus--the horse being four-footed--an oblation is offered for each of its feet.

As to this, they say, 'On the starting off of the horse he performs an offering; for the moon, doubtless, is the same as King Soma, the food of the gods: when, during that night (of new moon), he does not appear either in the east or in the west, then he comes to this world, and starts for this world.'

Now, when he performs the New-moon sacrifice, he thereby performs the (same) offering (as) on the starting of that (horse), and when he performs the Full-moon sacrifice he slaughters the sacrificial horse itself, and, having slaughtered it, he presents it to the gods. The other (real) horse-sacrifice they indeed perform (only) a year after (the starting offering), but this month (of the Full and New-moon sacrifice), revolving, makes up a year: thus the sacrificial horse comes to be slaughtered for him year after year.

Verily, then, for him who, knowing this, offers both the Agnihotra and the Full and New-moon sacrifices, they slaughter the sacrificial horse month by month; and month by month the Asvamedha is offered for him, and his Agnihotra and Full and New-moon sacrifices come to pass into the Asvamedha.