And the 118th mitzvah is that we are commanded that anyone who accidentally benefits from sanctified objects or eats holy food, i.e., terumah,1 must replace what he has eaten or the value of what he has benefited, and add an additional fifth.2

The source of this commandment is G‑d's statement,3 "For misappropriating something that was sacred, he must make full restitution, and add one fifth to it." It also says,4 "If a person inadvertently eats something holy [he must add one fifth...]".

The details of this mitzvah have been explained in tractate Me'ilah5 and also in tractate Terumos.6