הלכה על פאה 1:2

To leave over the corner of the field: To leave over a corner from the produce, as it is stated (Leviticus 19:10), "to the poor and to the stranger shall you leave them," after it mentioned, "you shall not finish the corner of your field" (Leviticus 19:9). And the understanding of stranger [here] is a righteous convert (see Sifra Kedoshim 3:4). And so [too], any "stranger" stated in [the context of] gifts to the poor - as behold, it is written about the second tithe (Deuteronomy 26:12), "to the stranger, to the orphan and to the widow." And that is certainly the righteous convert - when undifferentiated - as its witnesses (the orphan and the widow) are by its side. And the same is the case for all of the gifts to the poor. And nonetheless they, may their memory be blessed, said (Gittin 59b) that we do not prevent them from the poor of the idolaters, due to the ways of peace. And the content of the corner, is that a person leaves a little of his produce at the end of his field at the time that he reaps. And there is no measure to this remnant by Torah writ, but the Sages gave a measurement to the thing (Mishnah Peah 1:2), and it is one in sixty parts.