Leviticus 19:10 "And you shall not glean your vineyard,... you shall leave them for the poor and for the stranger"

A Jew must think about the needy not only in his field, but also in his vineyard.

As he is picking his clusters of ripened grapes, he must look out for those clusters of grapes that are not attached to a central stem or do not have the grapes lying one upon the other.

HaShem commands him to leave those clusters for the poor (see Negative Mitzvah 212).