"כתר...שמע ישראל ה' אלקינו ה' אחד"
“A crown given to you... Hear O Israel, G‑d our G‑d, G‑d is the One and Only.” (6:4)

QUESTION: Why is the Shema recited in the Kedushah of the Musaf Amidah on Shabbat and Yom Tov?

ANSWER: During the middle of the fifth century, the Persian king Yezdegerd II (see Zevachim 19a) issued a decree forbidding the daily recital of the Shema. His purpose was in order to eradicate belief in Hashem and his Oneness. To counteract this, the Sages inserted it into the Kedushah prayers.

The Talmudic sage Rav Ashi prayed for the abolishment of the decree, and miraculously a crocodile appeared in the king’s bedroom and swallowed him up in broad daylight, and the decree was nullified. The recital of Shema was thus reinstated, and removed from the Kedushah of Shacharit. However, in order that this miracle be remembered, it was left in the Kedushah of Musaf, since there is otherwise no reciting of the Shema during the prayer.

It is not mentioned, however, in the Rosh Chodesh Musaf Amidah, because Rosh Chodesh is often on a weekday when the attendance for public prayer is not so large and the goal of publicizing the miracle does not apply.

(שבולי הלקט אות מ"ה, ועי' לבוש סי' תכ"א)