Tosefta על ויקרא 19:23

A Gentile who was shouting, and said "Come and buy for yourselves! This is produce from newly broken land (=עזק, see Jastrow)! This is uncircumcised fruit (Lev. 19:23)! This is fourth-year fruit (Lev. 19:24)!" he is not to be trusted, because he is like one who praises [מקחו], but if he says "This is from such-and-such Samaritan," [or] "I bought this from such-and-such Gentile," [or] "This comes from my own," he is trusted to be stringent, the words of Rebbi. Rabban Shimon ben Gamaliel says, "We do not trust him, as the words of a Gentile make no difference (lit., do not go up and do not go down)."