דברים כא:ו מדרש: Sifrei Devarim

(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.