Rashi on Numbers 9:6/רש"י על במדבר ט׳:ו׳

1לפני משה ולפני אהרן [AND THERE WERE CERTAIN MEN, WHO … COULD NOT KEEP THE PASSOVER AND THEY CAME] BEFORE MOSES AND BEFORE AARON — At a time when both were sitting together in the House of Study they came and put this question to them. It would not be correct to say — as the words “”before Moses and before Aaron” might suggest — that they came to them one after the other, because if Moses did not know the reply whence could Aaron know it? (cf. Sifrei Bamidbar 68; Bava Batra 119b).