Demai, Chapter 2, Mishnah 2. This mishnah deals with the process by which an am ha'aretz - whose produce, when purchased from him, is considered demai - can become a chaver and a ne'eman, that is, a person who is trusted to separate tithes from his produce.
The conditions the ne'eman accepts upon himself:
"HaMekabel Alav Lihyos Ne'eman" - one who wishes to accept upon himself to be trusted regarding tithes, such that his produce will not be considered demai, must commit himself before witnesses to three things, and to refrain from one thing:
"Me'aser Es SheHu Ochel" - he eats only items from which tithes have been separated, and this includes anything he buys from an am ha'aretz, from which he separates tithes according to the laws of demai, and all the more so his own produce.
"Es SheHu Mocher" - anything he sells from his fields, he separates tithes from it.
"Es SheHu Loke'ach" - even from what he buys. For "that which he eats" would seem to already include the items he buys in order to eat, and therefore the intent here is one who buys in order to resell: even though this is not something he is going to eat but rather to sell, he is required to verify and to separate tithes from it.
"V'Eino Misare'ach Etzel Am HaAretz" - he does not accept hospitality from an am ha'aretz and eat at his table.
The dispute between Rabbi Yehudah and the Sages:
"Rabbi Yehudah Omer: Af HaMisare'ach Etzel Am HaAretz Ne'eman" - even one who did accept hospitality from an am ha'aretz remains trustworthy.
It should be emphasized that from the outset this person accepted upon himself not to eat at the home of an am ha'aretz, for among the conditions listed at the beginning of the mishnah it says "he tithes that which he eats" - whatever he is going to eat from an am ha'aretz, he will tithe. The discussion here concerns someone who accepted this upon himself and then failed, eating one time at the home of an am ha'aretz: is that enough to ruin his established status and standing? In Rabbi Yehudah's opinion, it is not enough to ruin his standing.
"Amru Lo: Al Atzmo Eino Ne'eman, Keitzad Yehei Ne'eman Al Shel Acherim?" - the Sages answered him: this person, before he formally became a ne'eman, was not trusted regarding himself; how, then, can he be trusted regarding the produce of others - to say about people who are not accepted as trustworthy that the tithes were taken from what he ate of theirs? On this basis they ruled that he is not trusted, and that this eating ruins his established status and standing, and he must once again accept the conditions upon himself anew.
In summary: in this mishnah we learned the conditions a person must accept upon himself in order to become a ne'eman: to tithe that which he eats, that which he sells and that which he buys, and not to accept hospitality from an am ha'aretz. We examined the dispute between Rabbi Yehudah and the Sages regarding one who failed and did accept hospitality from an am ha'aretz - in Rabbi Yehudah's opinion his established trustworthiness is preserved, while in the opinion of the Sages he is not trusted, since one who is not trusted regarding himself is not trusted regarding the produce of others, and he must once again accept the conditions upon himself anew.