Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.4.5

2.5.4.5

atha yatsāvitraḥ | dvādaśakapālo vāṣṭākapālo vā puroḍāśo bhavati savitā vai devānām prasavitā savitṛprasūtā haivainamaghnaṃstasmātsāvitro bhavati

Then as to why there is a cake on twelve, or eight, potsherds for Savitri. Savitri, indeed, is the impeller (prasavitri) of the gods; and impelled by Savitri they slew him: hence there is (a cake) for Savitri.