Reference for Bekhorot 71:81
שאני התם
R'Shizbi raised an objection: He who says to one who is not trustworthy with reference to tithing.<span class="x" onmousemove="('comment',' Who is known not to be observant as regards tithing.');"><sup>46</sup></span> 'Purchase on my behalf produce from one who is trustworthy<span class="x" onmousemove="('comment',' Not to purchase produce from an 'am ha-arez or, if he does so, to give dem'ai (v. Glos.) before selling it.');"><sup>47</sup></span> or from one who tithes', he is not believed.<span class="x" onmousemove="('comment',' On saying that he bought from a person trustworthy in these matters (Dem'ai IV, 5) .');"><sup>48</sup></span> Now why [is this so]? Let us adopt the principle that 'people are not presumed to tell a lie which is likely to be found out'? - The case is different there,