Mishnah Demai

He who purchases produce from one who is not [deemed] trustworthy in the matter of tithing, and forgot to tithe it, [and—Ed.] inquires of him [of the vendor] on the Sabbath, may eat at his [the vendor’s] word. [But] at nightfall at the close of the Sabbath, he should not eat until he has tithed [the produce].

[If] he did not find him [the vendor, on the Sabbath], [and if] someone else who is not [deemed] trustworthy in the matter of tithing said to him, “It is tithed,” he eats at his word. [But] at nightfall at the close of the Sabbath, he should not eat until he has tithed [the produce].

[Regarding] heave offering¹ of the tithe from demai produce which returned to its place [which fell back into the now-tithed demai produce from which is was originally separated, thus rendering the entire mixture prohibited to a non-priest]—R. Simeon of Sezur says, “Even on a weekday he inquires of him [the vendor] and eats at his word.”