Talmud for Sanhedrin 3:3
וְאֵלּוּ הֵן הַפְּסוּלִין, הַמְשַׂחֵק בְּקֻבְיָא, וְהַמַּלְוֶה בְרִבִּית, וּמַפְרִיחֵי יוֹנִים, וְסוֹחֲרֵי שְׁבִיעִית. אָמַר רַבִּי שִׁמְעוֹן, בִּתְחִלָּה הָיוּ קוֹרִין אוֹתָן אוֹסְפֵי שְׁבִיעִית, מִשֶּׁרַבּוּ הָאַנָּסִין, חָזְרוּ לִקְרוֹתָן סוֹחֲרֵי שְׁבִיעִית. אָמַר רַבִּי יְהוּדָה, אֵימָתַי, בִּזְמַן שֶׁאֵין לָהֶם אֻמָּנוּת אֶלָּא הִיא, אֲבָל יֵשׁ לָהֶן אֻמָּנוּת שֶׁלֹּא הִיא, כְּשֵׁרִין:
These are the ones who are invalidated [to judge and to testify]: One who games with kuvia (dice) (see Rosh Hashanah 1:8). [He is unfit to testify for he does not engage in "the habitation of the world." And one is forbidden to engage in his world in anything except Torah and lovingkindness, or in trade, craft, or work conducing to the habitation of the world.], one who lends on interest [Both the borrower and the lender are invalidated, it being ruled that both are in violation of a negative commandment], those who race pigeons [Some understand this as a form of gambling, viz.: "If your pigeon comes before my pigeon, I will give you, etc."; and some understand it as training pigeons to "abduct" other pigeons to their dove-cote, this being "theft by reason of (i.e., by violation of) 'the ways of peace,'" and not absolute theft], and those who trade in (produce of) shevi'ith (the sabbatical year), [of which it is written (Leviticus 25:6): "And the resting of the land shall be for you to eat" — and not for trade.] R. Shimon said: In the beginning they called them "the gatherers of shevi'ith" [The Gemara interprets R. Shimon thus: In the beginning they called them "the gatherers of shevi'ith." That is, those who gathered the fruits of shevi'ith for themselves were unfit to testify, just as those who traded in the fruits of shevi'ith.] With the increase of the "extorters" [those who requisitioned the king's portion, e.g., so many and so many korin of grain each year, so that they had to gather the produce of shevi'ith to fill the king's quota], they reverted to calling them "traders in shevi'ith" [i.e., they reverted to saying that "the traders" in the fruits of shevi'ith alone were unfit to testify, but those who gathered the produce of shevi'ith to give it to the king are fit to testify, since they do not gather it to store it for themselves. As to the halachah: Whoever commits a transgression punishable by judicial death penalty, kareth (cutting-off) or stripes, is unfit to testify, for one who has incurred the death penalty is called "an evildoer," viz. (Numbers 35:31): "...who is an evildoer (condemned) to die," as is one who has incurred stripes, viz. (Deuteronomy 25:2): "And it shall be, if liable to stripes is the evildoer," and it is written (Exodus 23:1): "Do not place your hand with an evildoer to be a false witness," which is expounded: "Do not make an evildoer a witness." And if he receives stripes, he reverts to his fitness, viz. (Deuteronomy 25:3): "And your brother (in receiving stripes) shall be demeaned before your eyes" — Once he has been smitten, he is as your brother. And if he took money unlawfully, even though he incurs neither the death penalty nor stripes, he is unfit to testify, e.g., a thief, a robber, and a lender on interest. And if he took money in violation of a rabbinic ordinance, he is unfit to testify by rabbinic ordinance, e.g., pigeon racers, and "forcers," who give money and take things that the owners do not wish to sell, charity collectors and tax collectors who take for themselves, and those who accept charity from gentiles in public — these and their like are unfit to testify by rabbinic ordinance. And their testimony is not invalid until they are exposed and publicized; but those who are unfit to testify by Torah law need not be thus exposed. And all who are unfit to testify, whether by Torah law or by rabbinic ordinance, if it can be ascertained that they have fully repented, and returned the money that they took unlawfully, and made a "fence" for themselves in the matter in which they sinned, so as not to repeat it, they return to their (state of) fitness. And those who game with kuvia, even though they are not guilty of theft even by rabbinic ordinance, they are unfit to testify, for they do not engage in "the habitation of the world" and have no fear of Heaven. And (they are thus unfit) only when they have no occupation but that, as per R. Yehudah (below). And this is the halachah. And when are they considered to have repented? When they break their dice and take it upon themselves not to game even gratis.] R. Yehudah said: When (are they unfit to testify)? When they have no occupation but that. But if they have an additional occupation, they are fit.