Positive commandment · Mitzvat Aseh #133 of 613
To separate challah [and give it] to the priest
להפריש חלה לכוהן
The verse · Bamidbar 15:20
רֵאשִׁית עֲרִסֹתֵכֶם חַלָּה תָּרִימוּ תְרוּמָה כִּתְרוּמַת גֹּרֶן כֵּן תָּרִימוּ אֹתָהּ
“You shall offer up a cake of the first of your dough for a gift: as you do the gift of the threshingfloor, so shall you set it apart.”
The Rambam’s words
To separate challah [and give it] to the priest, as [Numbers 15:20] states: "The first of your dough, the challah, you shall separate as an offering...."
להפריש חלה לכוהן, שנאמר: "ראשית, עריסותיכם--חלה, תרימו תרומה" (במדבר טו,כ).
Why · Sefer HaChinuch #385
Challah (dough-offering)
“It is from the roots of the commandment [that it is that] since the sustenance of a person is through food and most of the world will be sustained with bread, the Omnipresent desired to give us merit with a constant commandment in our bread, so that blessing should rest upon it through the commandment; and through it, we will receive merit for our souls.”
Who & when
“The commandment is practiced by males and females in the Land of Israel by Torah writ, as it is stated, “when you eat from the bread of the Land,” but specifically at the time that all of [the people of] Israel are there, meaning to say their majority.”
Where in the Mishneh Torah
Hilchot First Fruits and other Gifts to Priests Outside the Sanctuary הלכות ביכורים ושאר מתנות כהונה שבגבולין