Reference for Zevachim 230:10

תנאי היא דתניא (שמות יט, כב) וגם הכהנים הנגשים אל ה' יתקדשו ר' יהושע בן קרחה אומר זו פרישות בכורות רבי אומר זו פרישות נדב ואביהוא

Rabbi said: This intimated the separation of Nadab and Abihu.<span class="x" onmousemove="('comment',' Not the firstborn but actual priests are meant, viz., Nadab and Abihu, who became priests at Sinai. - Thus Rabbi holds that henceforth only the children of Aaron might act as priests, while R. Joshua b. Karhah maintains that the service was still performed by the firstborn.');">¹²</span> On the view that this meant the separation of Nadab and Abihu, it is well: hence it is written, This is that the Lord spoke, saying.'

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