Comentario sobre Números 15:20

ראשית ערסתכם OF THE FIRST OF YOUR DOUGH [YE SHALL HEAVE A CAKE] — This means: when you knead as much as is now your dough (ערסתכם), i.e. the quantity which you are accustomed to knead in the wilderness — and how much is this? You may learn it from the statements (Exodus 16:18) “And they measured with the Omer” and (Exodus 16:16) “An Omer a head”, — you shall set apart at its very beginning (ראשית), meaning, before you eat from it its first portion, one cake (חלה) shall you set apart, as a heave-offering in honour of the Lord (תרימו תרומה לה׳) (Eruvin 83a).

חלה תרימו תרומה; after the sin of the spies also the tithe (heaving) known as challah became a necessity in order for the individual homes of the Israelites to enjoy G’d’s blessing. The prophet Ezekiel spells this out (Ezekiel 44,30) when speaking about the temple etc. in the future, when he writes: עריסותיכם תתנו לכהן להניח ברכה אל ביתך, “you are to give from your kneading bowl to the priest so that a blessing will rest on your house.” This practice had already been displayed when Elijah, assisting the impoverished widow, commanded her to first give to him a small cake of the little flour she had left, as G’d had said that in such a case the jar of flour in her house would not become empty during the remainder of the famine. (Kings I 17,13-14)

כתרומת גורן, which is the first tithe of the grain harvest as we know from Deuteronomy 18,4. When converting the harvest to something edible, preparing the dough, the Priest is also to receive his share from the first part of the dough. (Ediyot 1,2 as well as Shabbat 15)