Shatapatha Brahmana 4.6.1.11

4.6.1.11

tasya dvādaśa prathamagarbhāḥ | paṣṭhauhyo dakṣiṇā dvādaśa vai māsāḥ saṃvatsarasya saṃvatsaraḥ prajāpatiḥ prajāpatiraṃśustadenam prajāpatiṃ karoti

Twelve heifers pregnant with their first calf are the priests' fee for this (graha). Now there are twelve months in the year, and Pragâpati is the year, and the Amsu is Pragâpati: thus he makes him (the sacrificer) to be Pragâpati.