Midrasch zu Schir haSchirim 4:6

Thereupon the Holy One, blessed be He, said to His ministering angels: Come, let us visit the sick. They replied: Master of the Universe, Lord, what is man, that Thou takest knowledge of him, or the son of man, that Thou makest account of him? (Ps. 144:3). Why should you go into an unclean place, into an area bathed in blood and defilement with an evil stench? He answered: Be assured, the blood of the prepuce is sweeter to Me than myrrh and frankincense. And thus Solomon sang: Until the day breatheth, and the shadows flee away, I will get me to the mountain of myrrh, and to the hill of frankincense (Song 4:6). This verse alludes to Abraham, who was called myrrh, as it is said: Myrrh, aloes, and cassia are all thy garments (Ps. 45:9).

(Numb. 5:15:) “Then he shall bring the offering for her, one-tenth an ephah of barley meal.” Why does she bring barley meal?¹⁹Sot. 2:1; cf. Sifre, to Numb. 5:15. Because she behaved in the manner of a beast with him, she therefore brings the food of a beast as her offering. (Ibid. cont.:) “He shall pour no oil upon it.” Why does he not pour oil upon it?²⁰Numb. R. 9:13. Because the oil is a light and is [also] called yitshar (a word which can mean "it will shine"); while this woman loves the darkness. Therefore there should be no light in her offering. [(Ibid., cont.:) Nor shall he put frankincense upon it.”] Why does he not put frankincense upon it? Because the frankincense is a remembrance of the matriarchs, [as stated] (in Cant. 4:6), “I will go unto the mountain of myrrh and unto the mound of frankincense,” while this woman has withdrawn from their ways. Her offering should [therefore] be a reminder of iniquity and not a reminder of merit.

Another explanation. Until the day breatheth (Song 4:6). This refers to judgment day, as it is said: For, behold, the day cometh, it burneth as a furnace; and all the proud, and all that work wickedness, shall be stubble; and the day that cometh shall set them ablaze, saith the Lord of hosts, that it shall leave them neither root nor branch (Mal. 3:19).