Bekhorot 30B

חשוד על המעשר, ומאן חכמים? רבי יהודה, וחד אמר: החשוד על המעשר חשוד על השביעית, ומאן חכמים? רבי מאיר.

is suspect with regard to tithe. And who are the rabbis referred to here as the Rabbis? It is R' Yehuda, as in his locale they treated the prohibition of produce of the Sabbatical Year stringently. And the other one says: One who is suspect with regard to tithe is suspect with regard to produce of the Sabbatical Year. And who are the rabbis referred to here as the Rabbis? It is R' Meir.

דתניא: עם הארץ שקיבל עליו דברי חבירות, ונחשד לדבר אחד ⁠— נחשד לכל התורה כולה, דברי רבי מאיר. וחכמים אומרים: אינו נחשד אלא לאותו דבר בלבד.

As it is taught in a baraita (Tosefta, Demai 2:4): With regard to an am ha’aretz, i.e., one who is unreliable with regard to ritual impurity and tithes, who accepts upon himself the commitment to observe the matters associated with ḥaver status, i.e., that he will be stringent in all matters observed by ḥaverim, including teruma, tithes, and ḥalla, and also undertake to eat only food that is ritually pure, and the rabbis accepted him as trustworthy but subsequently he was suspected with regard to one matter in which others saw him act improperly, he is suspected with regard to the entire Torah. This is the statement of R' Meir. And the Rabbis say: He is suspected only with regard to that particular matter.

הגר שקיבל עליו דברי תורה, אפילו נחשד לדבר אחד ⁠— הוי חשוד לכל התורה כולה, והרי הוא כישראל משומד. נפקא מינה, דאי קדיש ⁠— קידושיו קידושין.

It is also taught in a baraita (Tosefta, Demai 2:4): With regard to a convert who accepted upon himself upon his conversion matters of Torah, i.e., all of the mitzvot, even if he is suspect with regard to one matter alone, he is suspect with regard to the entire Torah, and he is considered like a Jewish transgressor [meshummad], who habitually transgresses the mitzvot. The Talmud explains that the practical difference resulting from the fact that he is considered like a Jewish transgressor is that if he betroths a woman, his betrothal is a valid betrothal, and they are married. Although he is suspect with regard to the entire Torah, he does not return to his prior non-Jew status.