Reference for Keritot 43:10

וזה לכם הטמא זה טמא ואין דם מהלכי שתים טמא אלא טהור

Said R'Zera in the name of Rab: This holds good only with regard to the heart of a fowl which is not as big as an olive's bulk in all; the heart of a beast, however, which comprises an olive's bulk, is forbidden and [whoso eats it] incurs the penalty of kareth.

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