Shatapatha Brahmana 11.2.5.4

11.2.5.4

sa yadāmāvāsyena yajate vivṛtta evāsyaitadiṣṭiṃ nirvapatyatha yatpaurṇamāsena yajate'śvamedhamevaitadālabhate tamālabhya devebhyaḥ prayacati saṃvatsare vā itaramaśvamedhamālabhanta eṣa vai māsaḥ pariplavamānaḥ saṃvatsaraṃ karoti tadasya saṃvatsare saṃvatsara evāśvamedha ālabdho bhavati

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.