Reference for Makkot 1:7
עַל פִּי שְׁנַיִם עֵדִים אוֹ שְׁלשָׁה עֵדִים יוּמַת הַמֵּת (שם יז), אִם מִתְקַיֶּמֶת הָעֵדוּת בִּשְׁנַיִם, לָמָּה פָרַט הַכָּתוּב בִּשְׁלשָׁה, אֶלָּא לְהַקִּישׁ שְׁלשָׁה לִשְׁנַיִם, מַה שְּׁלשָׁה מַזִּימִין אֶת הַשְּׁנַיִם, אַף הַשְּׁנַיִם יָזוֹמּוֹ אֶת הַשְּׁלשָׁה. וּמִנַּיִן אֲפִלּוּ מֵאָה, תַּלְמוּד לוֹמַר, עֵדִים. רַבִּי שִׁמְעוֹן אוֹמֵר, מַה שְּׁנַיִם אֵינָן נֶהֱרָגִין עַד שֶׁיְּהוּ שְׁנֵיהֶם זוֹמְמִין, אַף שְׁלשָׁה אֵינָן נֶהֱרָגִין עַד שֶׁיִּהְיוּ שְׁלָשְׁתָּן זוֹמְמִין. וּמִנַּיִן אֲפִלּוּ מֵאָה, תַּלְמוּד לוֹמַר, עֵדִים. רַבִּי עֲקִיבָא אוֹמֵר, לֹא בָא הַשְּׁלִישִׁי אֶלָּא לְהַחְמִיר עָלָיו וְלַעֲשׂוֹת דִּינוֹ כַיּוֹצֵא בָאֵלּוּ. וְאִם כֵּן עָנַשׁ הַכָּתוּב לַנִּטְפָּל לְעוֹבְרֵי עֲבֵרָה כְעוֹבְרֵי עֲבֵרָה, עַל אַחַת כַּמָּה וְכַמָּה יְשַׁלֵּם שָׂכָר לַנִּטְפָּל לְעוֹשֵׂי מִצְוָה כְעוֹשֵׂי מִצְוָה:
(Deuteronomy 17:6): "By the word of two witnesses or three witnesses shall the dead one be put to death." If the testimony stands with two, why does Scripture indicate three? To liken three to two, viz. Just as three can render two, zomemin, so two can render three, zomemin. And whence do we derive [that two can render zomemin] even a hundred? From (Ibid.): "witnesses" (i.e., any number). R. Shimon says: Just as two are not killed until both are rendered zomemin, [it being written (Ibid. 19:18): "And, behold, a false witness is the witness," concerning which the master said: Wherever "witness" is written, two (witnesses) are understood, unless it is explicitly stated "one"], so, three are not killed until the three are rendered zomemin. [This, if each testifies immediately upon the conclusion of the other's testimony. But if two testify and after some time, the others do, they are two distinct sets of witnesses in all respects.] And whence do we derive (that this is so) even (with) a hundred? From: "witnesses." R. Akiva says: The third (i.e., "three witnesses") comes only to (tell us to) be stringent with him and to equate his judgment with that of the others, [that one not say: Since even without the third, the testimony (of the others) would stand, the judgment of hazamah should not apply to him. The verse apprises us (that this is not so) that he, too, is judged.]. If Scripture thus punished the accessory to transgressors as the transgressors themselves, how much more so will the accessories to the doers of a mitzvah be rewarded as the doers of the mitzvah themselves! [For "His measure for good is greater than that for punishment."]