Commentary for Berakhot 20:9
ומי לית ליה לחזקיהו (שמות יב, ב) החדש הזה לכם ראש חדשים זה ניסן ואין אחר ניסן
Did not Hezekiah know of the teaching, "This month shall be unto you the beginning of months" (Exod. xii, 2), i.e. this month and none other shall be Nisan?
Abraham Cohen Footnotes to the English Translation of Masechet Berakhot
I.e. the month having once been proclaimed Nisan, it cannot be intercalated.
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