Śatapatha Brāhmaṇa ("of a hundred paths") 13.2.5.1

13.2.5.1

prajāpatiraśvamedhamasṛjata so'smātsṛṣṭaḥ parāṅaitsa paṅktirbhūtvā saṃvatsaram prāviśatte'rdhamāsā abhavaṃstam pañcadaśibhiranuprāyuṅkta tamāpnottamāptvā pañcadaśibhiravārunddhārdhamāsānāṃ vā eṣā pratimā yatpañcadaśino yatpañcadaśina ālabhate'rdhamāsāneva tairyajamāno'varunddhe

Pragâpati poured forth the life-sap of the horse (asva-medha); when poured forth it went from him. Having become fivefold it entered the year, and they (the five parts) became those half-months. He followed it up by means of the fifteenfold (sets of victims), and found it; and having found it, he took possession of it by means of the fifteenfold ones; for, indeed, they--to wit, the fifteenfold (sets)--are a symbol of the half-months, and when he seizes the fifteenfold ones, it is the half-months the Sacrificer thereby takes possession of.