Negative commandment · Lo Ta’aseh #104 of 613
Not to place oil on the meal offering brought by a sotah
שלא ייתן שמן במנחת סוטה
The verse · Bamidbar 5:15
וְהֵבִיא הָאִישׁ אֶת־אִשְׁתּוֹ אֶל־הַכֹּהֵן וְהֵבִיא אֶת־קׇרְבָּנָהּ עָלֶיהָ עֲשִׂירִת הָאֵיפָה קֶמַח שְׂעֹרִים לֹא־יִצֹק עָלָיו שֶׁמֶן וְלֹא־יִתֵּן עָלָיו לְבֹנָה כִּי־מִנְחַת קְנָאֹת הוּא מִנְחַת זִכָּרוֹן מַזְכֶּרֶת עָוֺן
“then shall the man bring his wife to the priest, and he shall bring her offering for her, the tenth part of an efa of barley meal; he shall pour no oil upon it, nor put frankincense on it; for it is an offering of jealousy, an offering of memorial, bringing iniquity to remembrance.”
The Rambam’s words
Not to place oil on the meal offering brought by a sotah, as [Numbers 5:15] states: "Do not pour oil upon it."
שלא ייתן שמן במנחת סוטה, שנאמר "לא ייצוק עליו שמן" (במדבר ה,טו).
Why · Sefer HaChinuch #366
That we not put oil on the sotah sacrifice
“From the roots of the commandment is that which they, may their memory be blessed said (Bamidbar Rabbah 9:13) [that it is] so that her sacrifice not be beautiful.”
Who & when
“And it is practiced at the time of the Temple.”
Where in the Mishneh Torah
Hilchot Woman Suspected of Infidelity הלכות סוטה