Makkos, chapter 3, mishnah 15. Having finished discussing the manner in which the lashes are administered, the mishnah turns to what happens after the lashes have been received, and from there to the reward awaiting one who refrains from sin.
"Kol chayavei kerisos shelaku - niftaru yedei kerisasan":
All those who are liable to the punishment of kares - to be cut off from Heaven and not to merit the life of the World to Come - if they received lashes in court, they are released from their punishment of kares. From where do we know this? "Shene'emar: 'venikleh achicha le'einecha'" - that is, your brother becomes lowered and degraded in your eyes. Since the verse calls the one who was lashed "your brother," we learn that after he has been lashed he is once again like your brother, a full member of the Jewish people, and he is no longer subject to kares. These are the words of Rabbi Chananya ben Gamliel.
The measure of good is greater than the measure of punishment:
Rabbi Chananya ben Gamliel continues with a further idea connected to this: "Mah im ha'over aveirah achas nosen nafsho aleha" - if a person who transgressed a single sin that carries the penalty of kares has his soul taken from him and does not merit eternal life because of it, "ha'oseh mitzvah achas - al achas kamah vekamah shetinasen lo nafsho": one who fulfills a mitzvah, even a single mitzvah, will certainly merit a great eternal reward for his soul. The principle behind this: the measure of good, with which the Holy One, blessed is He, measures out good to people, is greater than the measure of punishment.
Rabbi Shimon: "He learns it from its own place":
Rabbi Shimon learns this from the passage that deals with the penalties of kares - the passage of forbidden relations, in which more cases of kares are concentrated than anywhere else. From that very passage the point can be learned:
"Shene'emar: 'venichresu hanefashos ha'osos'" - anyone who transgresses one of the prohibitions of forbidden relations becomes liable to kares.
"Ve'omer: 'asher ya'aseh osam ha'adam vachai bahem'" - at the beginning of that same passage, in the introduction to the prohibitions that follow, it says that a person "shall do them" and live by them. And what is the "doing" spoken of here? His very abstaining from these sins is counted for him as a mitzvah that he performed.
From here: "Kol hayoshev velo avar aveirah - nosnin lo sachar ke'oseh mitzvah". Even though he did nothing at all and remained in a passive state, he is given reward as though he had actively fulfilled a mitzvah.
Rabbi Shimon bar Rabbi: an a fortiori argument from blood:
Rabbi Shimon, the son of Rabbi, brings another illustration of this idea: "Rak chazak levilti achol hadam" - a person is required to strengthen himself not to eat, or more precisely not to drink, the blood, "ki hadam hu hanefesh" - the blood is the life force of the animal. And from here an a fortiori argument: "Umah im hadam shenafsho shel adam kotzah bo - haporesh mimenu mekabel sachar", for blood is something that a person's soul finds revolting to eat, and nevertheless one who holds himself back from it receives reward for doing so.
"Gezel ve'arayos shenafsho shel adam misavah lahen umechamdasan - haporesh mehen al achas kamah vekamah" - regarding sins that a person craves and desires, one who separates himself from them and guards himself from doing them will surely receive reward: "sheyizkeh lo uledorosav uledorei dorosav ad sof kol hadoros". This merit will stand for him and for his descendants after him until the end of all generations. If for refraining from blood, which a person has no desire to drink at all, one receives reward, then all the more so with things that a person does crave and from which he holds himself back, that the merit will stand for him for all the generations to come.
In summary: In this Mishnah we learned that those who are liable to kareis and received lashes in beis din are exempted from the punishment of kareis and return to being "achicha" - your brother, in every respect; that the measure of good is greater than the measure of punishment, and therefore the reward of one who performs a single mitzvah is greater than the punishment of one who transgresses a single sin; that one who refrains from a sin receives reward like one who performs a mitzvah, as derived from the verse "asher yaaseh osam ha'adam vachai bahem" - which a person shall do and live by them; and finally, through a kal vachomer from blood, that one who separates himself from sins that his soul craves earns reward for himself and for his descendants until the end of all generations.