Sifrei Devarim 209

וכל זקני העיר ההוא. אפילו הם מאה. לפי שאמרנו למעלה ג' וחמשה, יכול אף כאן כן? ת"ל ועל זקני העיר ההוא, ואפילו הם מאה.

(Devarim 21:6) "And all the elders of that city": Even if they are a hundred. Because we said above ([21:2] re the measuring), three or five, I might think that this is so here, too. (in respect to the washing of the hands); it is, therefore, written "and all the judges of that city" — even if they are a hundred.

ירחצו את ידיהם על העגלה הערופה בנחל. על מקום עריפתה של עגלה. יכול יעלו מן הנחל ויאמרו? ת"ל בנחל, שתהיה (אמירתם וענייתם) [רחיצתם ואמירתם] בנחל.

"they shall wash their hands over the heifer of the broken neck in the river-bed": on the spot where its neck was broken. I might think that they may go up from the river-bed; it is, therefore, written "in the river-bed." (Ibid. 7) "And they shall answer and they shall say": Their washing and their saying must be in the river-bed.