Śatapatha Brāhmaṇa ("of a hundred paths") 9.3.1.1

9.3.1.1

athāto vaiśvānaraṃ juhoti | atraiṣa sarvo 'gniḥ saṃskṛtaḥ sa eṣo 'tra vaiśvānaro devatā tasmā etaddhavirjuhoti tadenaṃ haviṣā devatāṃ karoti yasyai vai devatāyai havirgṛhyate sā devatā na sā yasyai na gṛhyate dvādaśakapālo dvādaśa māsāḥ saṃvatsaraḥ saṃvatsaro vaiśvānaraḥ

He then offers the Vaisvânara (cake). That Agni has now been completely restored; he now is that deity, (Agni) Vaisvânara (belonging to all men): to him he offers this oblation, and by the oblation he makes him a deity, for whatever deity an oblation is prepared that is a deity, but not one for whom no (oblation) is prepared. It is one of twelve potsherds: twelve months are a year, and Vaisvânara is the year.