Reference for Megillah 45:3

מני אי ר' ישמעאל קשיא תוספת אי ר"ע קשיא ששה ושבעה

Whom [does the Mishnah follow]? If R. Ishmael, it conflicts with him over the additional number, if R. Akiva, it conflicts with him over the question of six and seven!

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