I might think that he may eat a regular meal in the field (without tithing); it is, therefore, written "the field" (i.e., he must tithe it even there.)
And whence is it derived (that he must tithe) even what he sows? From "your seed."
And whence is it derived (that he must tithe) even what he stores? From "produce."
And whence do we derive (the same for) what he sells? From "which goes forth from the field." They said: Why were the shops of Bnei Chanan laid waste two years before (the destruction of) Eretz Yisrael? Because they exempted their fruits from the tithe, expounding "Tithe shall you tithe … and you shall eat" — and not what you sell; "the produce of your seed" — and not (that of) the buyer.