Sifrei Devarim 207

והורידו זקני העיר ההיא. מצוה בזקני העיר ההיא.

(Devarim 21:4) "And the elders of that city shall bring the heifer down": I might think that this is (only) a mitzvah, (but not a categorical requirement); it is, therefore, written again "the elders of that city," after having been written before (verse 3), to make it a categorical requirement. From here they ruled: If the body were found near the border or near a city inhabited mostly by gentiles or near a city lacking a beth-din, they would not measure.

אל נחל איתן. (ריש פרה) כמשמעו קשה. ואף על פי שאינה איתן, כשר.

("And they shall bring the heifer down) to a hard river-bed": And whence is it derived that it is valid even if it is not hard? From (Ibid.) "in the river bed" — even if it is not hard.

עורפין אותה בקופיץ מאחריה, ומקומה אסור מלזרוע ומלעבוד. יכול יהא אסור מלהדוק שם פשתן ומלנקר שם את האבנים? ת"ל לא יזרע. זריעה בכלל היתה, ולמה יצאת, להקיש אליה - מה זריעה מיוחדת שהוא (חובת קרקע) [בקרקע], יצאו אלו שאינם (חובת קרקע) [בקרקע].

("a hard river-bed) which shall not be worked": I might think that it is forbidden to comb flax there and to chisel stones there; it is, therefore, written "and which shall not be sowed." Sowing was included in all (labor). Why was it singled out? To make it a paradigm, viz.: Just as sowing is distinctly in the soil, so, all (such labors are forbidden), to exclude those which are not labors of the soil.

וערפו שם. נאמר כאן "עריפה" ונאמן (אולי "נאמר") להלן (שמות יג) "עריפה". מה "עריפה" האמור (להלן) [כאן], עורפים בקופיץ מאחריה ואסור בהנאה וקוברו; אף "עריפה" האמורה (כאן) [להלן], עורפו בקופיץ מאחוריו וקוברו ואסור בהנאה.

"and they shall break there the neck of the heifer": It is written here "breaking," and elsewhere (Vayikra 5:18) "breaking." Just as there, he breaks the neck with a hatchet from behind, so, here. "and they shall break there the neck": It is written here "there," and, elsewhere, "there." Just as there, he buries it and it is forbidden to derive benefit from it, so, here. [("a hard river-bed) which shall not be worked": I might think that it is forbidden to comb flax there and to chisel stones there; it is, therefore, written "and which shall not be sowed." Sowing was included in all (labor). Why was it singled out? To make it a paradigm, viz.: Just as sowing is distinctly in the soil, so, all (such labors are forbidden), to exclude those which are not labors of the soil.]

בנחל. אע"פ שאינו איתן.

"in the river bed" — even if it is not hard.