וּבַחֲמִשָּׁ֨ה עָשָׂ֥ר יוֹם֙ לַחֹ֣דֶשׁ הַזֶּ֔ה חַ֥ג הַמַּצּ֖וֹת לַיהוָ֑ה שִׁבְעַ֥ת יָמִ֖ים מַצּ֥וֹת תֹּאכֵֽלוּ׃
And on the fifteenth day of the same month is the feast of unleavened bread unto the LORD; seven days ye shall eat unleavened bread.
Mekhilta d'Rabbi Yishmael
R. Yoshiyah says: whence is it derived that the year is intercalated only in the Great Beth-din in Jerusalem? From "It is the first for you … Speak (you, Moses and Aaron, the leaders of the generation [akin to the Great Sanhedrin]), etc."