Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 203
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 203, with English translation by Maimonides (Rambam).
שלא יאכל ענבים לחים, שנאמר "וענבים לחים . . . לא יאכל" (במדבר ו,ג).
[For a Nazarite] not to eat fresh grapes, as [Numbers, ibid.] states: "[He shall not eat] fresh... grapes."