Positive commandment · Mitzvat Aseh #71 of 613
A definite guilt-offering
להקריב השוגג במעילה, או החוטא בגזילה, או בשפחה חרופה, או שכפר בפיקדון ונשבע; מביא קרבן אשם, וזה הוא הנקרא אשם ודאי
The verse · Vayikra 5:25
וְאֶת־אֲשָׁמוֹ יָבִיא לַה׳ אַיִל תָּמִים מִן־הַצֹּאן בְּעֶרְכְּךָ לְאָשָׁם אֶל־הַכֹּהֵן
“and he shall bring his guilt offering to the Lord, a ram without blemish out of the flock, according to the value of a guilt offering, to the priest:”
The Rambam’s words
For [the following individuals:] a person who unknowingly used sacred property, a person who sinned by stealing, one [who had relations with] a maidservant designated for another person, or one who denied possession of an entrusted object and took a [false] oath, to bring a guilt offering. This is referred to as "the unconditional guilt offering."
להקריב השוגג במעילה, או החוטא בגזילה, או בשפחה חרופה, או שכפר בפיקדון ונשבע; מביא קרבן אשם, וזה הוא הנקרא אשם ודאי.
Why · Sefer HaChinuch #129
A definite guilt-offering
“It is from the roots of the commandment [that] a person not think that even though the matter of the prohibition of robbery is rectified by a positive commandment, as it is stated (Leviticus 5:23), “and return the theft” — that each one should go and rob what he wants from his fellow and imagine in his heart to say that when he has it and he returns the theft, his iniquity will be atoned and he will be purified from it; and behold, it will be as if he never did it.”
Who & when
“And the commandment of the liability for this sacrifice is practiced at the time of the [Temple] by males and females.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה