Sefer HaChinuch 217 ספר החינוך רי״ז

To not finish the corner in the field: Not to harvest all of what is planted, but rather to leave a remnant from it in the edge of the field for the poor, as it is stated (Leviticus 23:22), “you shall not finish the corner of your field in your harvesting.” And this negative commandment is rectified by a positive commandment, as it is stated (Leviticus 19:10), “to the poor and to the stranger shall you leave them, etc.” — meaning to say, that if he transgressed and harvested all of the field, that he give the measure of the corner to the poor from that which is harvested. And that is rabbinically one in sixty parts, as they obligated us so; but from the Torah, it has no measure, as we learned in Mishnah Peah 1:1, “These are the things that do not have a measure: the corner, etc.”

I have written above what I have known about the roots of the commandment, its laws and all of its content on the positive commandment in this Order (Sefer HaChinukh 216).