עד אחד. זֶה בָנָה אָב – כָּל עֵד שֶׁבַּתּוֹרָה שְׁנַיִם אֶלָּא אִם כֵּן פֵּרֵט לְךָ בוֹ אֶחָד (סנהדרין ל'): עד אחד ONE WITNESS [SHALL NOT RISE UP AGAINST A MAN FOR ANY INIQUITY] — This is the classic passage from which the general principle is derived that wherever the term “witness” (עד) occurs in the Torah, it means two witnesses, unless it specifically mentions in connection with it the word אחד (Sanhedrin 30a).
לכל עון ולכל חטאת. לִהְיוֹת חֲבֵרוֹ נֶעֱנָשׁ עַל עֵדוּתוֹ, לֹא עֹנֶשׁ גּוּף וְלֹא עֹנֶשׁ מָמוֹן, אֲבָל קָם הוּא לִשְׁבוּעָה; אָמַר לַחֲבֵרוֹ תֵּן לִי מָנֶה שֶׁהִלְוִיתִיךָ, אָמַר לוֹ אֵין לְךָ בְיָדִי כְּלוּם וְעֵד אֶחָד מְעִידוֹ שֶׁיֵּשׁ לוֹ, חַיָּב לִשָּׁבַע לוֹ (שבועות מ'): לכל עון ולכל חטאת [ONE WITNESS SHALL NOT RISE UP AGAINST A PERSON] FOR ANY INIQUITY OR ANY SIN — i.e. that his fellow man should be punished on account of his evidence either by bodily punishment or by a monetary punishment; but he may rise up against his fellow men to compel him to take an oath. If, for instance, one says to his fellow, “Return me the maneh which I lent you”, and he replies “I have nothing of yours in my possession”, and one witness testifies that he has, he (the defendant) must take an oath to satisfy him (Shevuot 40a; cf. Sifrei Devarim 188:9).
על פי שני עדים. וְלֹא שֶׁיִּכְתְּבוּ עֵדוּתָם בְּאִגֶּרֶת וְיִשְׁלְחוּ לְבֵית דִּין, וְלֹא שֶׁיַּעֲמֹד תֻּרְגְּמָן בֵּין הָעֵדִים וּבֵין הַדַּיָּנִים (ספרי): על פי שני עדים [AT THE MOUTH OF TWO WITNESSES ... SHALL THE MATTER BE ESTABLISHED] — at their mouth, but not that they may write their evidence down in a letter and send it to the court, nor that an interpreter stand between the witnesses and the judges (Sifrei Devarim 188:10).