Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.4.3

2.5.4.3

sa yadāgneyo 'ṣṭākapālaḥ puroḍāśo bhavati | agninā ha vā enaṃ tejasāghnantsa tejo 'gnirnāvyathata tasmādāgneyo bhavati

Now as to why there is a cake on eight potsherds for Agni. With Agni, (shaped into) a sharp point (tegas), indeed, they (the gods) slew him (Vritra); and Agni, that sharp point, swerved not: hence there is (a cake) for Agni.