Rashi on Leviticus 5:11/רש"י על ויקרא ה׳:י״א

1כי חטאת הוא [HE SHALL PUT NO OIL UPON IT, NEITHER SHALL HE PUT ANY FRANKINCENSE THEREON:] FOR IT IS A SIN-OFFERING — a sinner’s offering, and therefore it is not proper that his offering should be embellished by oil and frankincense (Menachot 6a).