Rashi on Numbers 15:20/רש"י על במדבר ט״ו:כ׳

1ראשית 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).

2חלה is tourteau in O. F., (Engl = cake).

3כתרומת גרן EVEN AS THE HEAVE OFFERING OF THE THRESHING FLOOR for which no minimum quantity is stated in the Torah, so shall ye heave it, and not as the “Heave-offering of the Tithe (תרומת מעשר, the priest’s share in the Levite’s tithe) for which a minimum quantity is stated (cf. Numbers 18:26) (Sifrei Bamidbar 110:1). — The Sages, however have stipulated a minimum quantity for it (the portion set aside) — for a private person (lit., the owner of a house) one twenty-fourth, and for the baker one forty-eighth of the dough (Sifrei Bamidbar 110:1; Mishnah Challah 2:7).