Sefer HaChinuch 367 ספר החינוך שס״ז

Not to place frankincense on the sotah sacrifice: Not to place frankincense on the sotah sacrifice, as it is stated (Numbers 5:15), “and he shall not put frankincense on it.”

From the roots of the commandment is that which we have written about oil, as frankincense is a beautification of the sacrifice and a pleasant smell. And hence it is not fitting for a sinful woman to beautify her sacrifice. And they, may their memory be blessed, also said about this (Bamidbar Rabbah 9:13) that the proper and modest matriarchs were called frankincense, as it is stated (Song of Songs 4:6), “to the hill of frankincense” — and this one separated from their ways. And this commandment is also elucidated in the Scripture, and its matter is written in its “colleague” — there is no need to write at length about it.