Numbers 15:21

מראשית ערסתיכם OF THE FIRST OF YOUR DOUGH [YE SHALL GIVE UNTO THE LORD A HEAVE OFFERING] — Why is this stated at all? (What does it add to the command in v. 20)? Because it states there ראשית ערסתכם, I might understand this to mean that only the first amongst several doughs which you are kneading at the same time shall you set aside entirely as a heave-offering and all the other doughs are exempt! It therefore states here מראשית “of (some of) the first of the dough”, implying part of it but not all of it (not an entire dough) (Sifrei Bamidbar 110:2). תתנו לה׳ תרומה YE SHALL GIVE UNTO THE LORD A HEAVE OFFERING — Because we have not heard any mention of a definite minimum quantity for the Challah it is stated here “ye shall give”, implying that it shall contain sufficient to constitute a “gift” (Sifrei Bamidbar 110:2).

מֵרֵאשִׁית֙ עֲרִסֹ֣תֵיכֶ֔ם תִּתְּנ֥וּ לַיהֹוָ֖ה תְּרוּמָ֑ה לְדֹרֹ֖תֵיכֶֽם׃ {ס}        

Of the first of your dough you shall give to the Lord an offering throughout your generations.