Negative commandment · Lo Ta’aseh #152 of 613
Not to use the proceeds [from the redemption of] the second tithe for anything aside from food and drink
שלא להוציא דמי מעשר שני בשאר דברים שאין בהם אכילה ושתייה
The verse · Devarim 26:14
לֹא־אָכַלְתִּי בְאֹנִי מִמֶּנּוּ וְלֹא־בִעַרְתִּי מִמֶּנּוּ בְּטָמֵא וְלֹא־נָתַתִּי מִמֶּנּוּ לְמֵת שָׁמַעְתִּי בְּקוֹל ה׳ אֱלֹהָי עָשִׂיתִי כְּכֹל אֲשֶׁר צִוִּיתָנִי
“I have not eaten of it in my mourning, neither have I consumed any part of it when unclean, nor given of it for the dead: but I have hearkened to the voice of the Lord my God, and have done according to all that thou hast commanded me.”
The Rambam’s words
Not to use the proceeds [from the redemption of] the second tithe for anything aside from food and drink, as [Deuteronomy, ibid.] states: "I have not used it for the dead." Anything that is not associated with the needs of a living body is referred to as "used for the dead." his interpretation of the verse appears to be original and not quoted from other sources.
שלא להוציא דמי מעשר שני בשאר דברים שאין בהם אכילה ושתייה, שנאמר "ולא נתתי ממנו, למת" (דברים כו,יד); וכל שהוא חוץ לצורכי הגוף החי, "נתתי ממנו, למת" אקרא בו.
Who & when
“And this prohibition is practiced by males and females at the time of the [Temple], since we then had tithes from Torah writ.”
Where in the Mishneh Torah
Hilchot Second Tithes and Fourth Year's Fruit הלכות מעשר שני ונטע רבעי