Shatapatha Brahmana 4.3.1.5

4.3.1.5

tānvai dvādaśa gṛhṇīyāt | dvādaśa vai māsāḥ saṃvatsarasya tasmāddvādaśa gṛhṇīyādatho api trayodaśa gṛhṇīyādasti trayodaśo māsa iti dvādaśa tveva gṛhṇīyādeṣaiva sampat

Let him draw twelve of them,--twelve months there are. in the year: therefore he should draw twelve (cups of Soma). But he may also draw thirteen, for, they say, there is a thirteenth month. Let him nevertheless draw twelve only, for such is completeness.