Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 119
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 119, with English translation by Maimonides (Rambam).
להיות נטע רבעי קודש, שנאמר: "יִהְיֶה כָּל פִּרְיוֹ קֹדֶשׁ הִלּוּלִים" (ויקרא יט, כד).
For the produce of the fourth year to be sacred, as [Leviticus 19:24] states: "Its produce shall be sacred, [an object of] praise to God...."