Sifrei on Deuteronomy
לא תשיך לאחיך. אין לי אלא לוה, מלוה מנין? ת"ל אל תקח מאתו נשך ותרבית.
(Devarim 23:20) "You shall not give interest to your brother": This tells me only of the borrower (that he should not give interest.) Whence do I derive (the same for) the lender (that he should not take interest)? From (Vayikra 25:36) "You shall not take from him interest."
מכלל שנאמר "כספך" - ולא כסף אחרים, "אכלך" - ולא אוכל אחרים. או "כספך" - ולא כסף מעשה, "אכלך" - ולא אוכל בהמה? כשהוא אומר "נשך כסף", לרבות כסף מעשר. "נשך אוכל", לרבות אוכל בהמה. אין לי אלא "נשך כסף נשך אוכל", מנין לרבות כל דבר? ת"ל "נשך כל דבר אשר ישך".
(Vayikra 25:37: "Your money you shall not give him on interest, and on increase you shall not give your food."): From "your money," I derive: your money, but not that of others (i.e., gentiles); "your food," but not that of others. — But perhaps (the inference should be) "your money," but not that of ma'aser (the tithe); "your food," but not that of beasts! — (No, for "interest of money" (Devarim, Ibid.) includes money of ma'aser (as interdicted); "interest of food" includes food of beasts. This tells me only of interest of money and interest of food (as interdicted). Whence do I derive the same for any kind of interest? From (Ibid.) "interest of anything that can be (construed as) interest."
ר' שמעון אומר, מנין שלא יאמר לו, "צא ושאול בשלום פלוני" או "דע אם בא איש פלוני ממקומו"? ת"ל (נשך) כל דבר (אשר ישך).
R. Shimon says: Whence is it derived that he (the lender) should not tell him (the borrower): Go and greet that man, or: Find out if that man has come from his place? From "interest of any thing ("devar," like "dibbur" [speech]).