Daily Mitzvah (Rambam)

"If a person sins and commits one of the commandments of G‑d which may not be committed, but he does not know, he is guilty, and he shall bear his transgression. He shall bring an unblemished ram from the flock, with the value for a guilt offering, to the kohen. The kohen shall then make atonement for his unintentional sin which he committed and did not know."—Leviticus 5:17-18.

One who is uncertain whether he committed a sin whose transgression would require him to bring a Sin Offering – e.g., two pieces of fat were before him, one permitted and one forbidden; he consumed one of them and the other was then lost (so that it was impossible to identify whether it was of the forbidden or permitted variety) – he must then bring an Asham Taluy (a "Doubtful Guilt Offering").

(If afterwards it was conclusively ascertained that he ate of the forbidden fats, he must bring a standard Sin Offering [even if he has already brought his Asham].)

And the 70th mitzvah is that we are commanded to bring a sacrifice called an asham talui, ("suspended guilt-offering") when in doubt regarding a sin [as to whether or not it has been committed. The type of sin referred to is one] which when done intentionally is punishable by kares, and when done unintentionally, one must bring a fixed sin-offering.1

An example of a doubt which would necessitate an asham talui is as follows: a person had two pieces of fat lying before him — one from the kidneys [and therefore prohibited] and the other from the heart [which is permitted]. The person ate one of them and the other was either eaten by someone else or lost. The person is now in doubt as to whether he ate the permissible piece or the prohibited piece. In this case, because of his doubt, he must bring a sacrifice in order to obtain atonement. This sacrifice is known as an asham talui.

If later on it became clear that he ate fat from the kidneys, we now know that he definitely committed an unintentional transgression and must bring a fixed sin-offering.

The verse which speaks of this offering is G‑d's statement in the Torah portion Leviticus,2 "If a person sins by violating one of G‑d's prohibitions without knowing [for sure], he still bears responsibility. He must bring an unblemished ram with the prescribed value to the kohen as a guilt offering. The kohen shall then make atonement for the inadvertent sin that the person committed without knowing." [The phrase "without knowing"] refers to his not knowing whether he actually did or did not unintentionally perform the transgression. Our Sages3 refer to this as lo hodah.

The details of this mitzvah are explained in tractate Kerisus.4