Rashi on Deuteronomy 14:29/רש"י על דברים י״ד:כ״ט

1ובא הלוי AND THE LEVITE SHALL COME and take the “First Tithe” which you have failed to give him,

2והגר והיתום AND THE STRANGER AND THE FATHERLESS [SHALL COME] and take the “Second Tithe”, which belongs to the poor in that year, and you shall not eat it yourself in Jerusalem, as you were bound to eat the “Second Tithe” of the first two years,

3ואכלו ושבעו AND THEY SHALL EAT AND BE SATISFIED — Give them sufficient to satisfy them. From here they (the Rabbis) derived the law: one must give the poor in the barn no less than half a kab of wheat or a kab of barley (Sifrei Devarim 110:3). As for you, go up to Jerusalem with the tithe (i.e. מעשר שני) of the first and the second years which thou hast delayed, and make the confession there: “I have removed the hallowed things from the house” — as it is stated in the section beginning with the words כי תכלה לעשר (Deuteronomy 26:12).