Śatapatha Brāhmaṇa ("of a hundred paths") 5.5.2.2

5.5.2.2

tāni vai dvādaśa bhavanti | dvādaśa vai māsāḥ saṃvatsarasya tasmāddvādaśa bhavanti māsi-māsi yajetetyāhuḥ ko veda manuṣyasya tasmānna māsi-māsi yajeta śamyāparāvyādhe śamyāparāvyādha eva ṣaḍbhiryajate prāṅ yānatha punarāvṛttaḥ śamyāparāvyādhe śamyāparāvyādha eva ṣaḍbhiryajate

There are twelve of these (oblations), for there are twelve months in the year: that is why there are twelve. 'Let him make offering month by month,' they say. Who knows about (the life of) man? Let him therefore not make offering month by month. Moving eastward he offers six of them each at the distance of the yoke-pin's throw from the other; and then turning backward he offers six, each at a yoke-pin's throw from the other.