Rashi on Numbers 9:4/רש"י על במדבר ט׳:ד׳

1וידבר משה וגו׳ AND MOSES SPAKE [TO THE CHILDREN OF ISRAEL THAT THEY SHOULD KEEP THE PASSOVER] — Why is this stated at all? Has it not already been said, (Leviticus 23:44) “And Moses declared [unto the children of Israel] the appointed festivals of the Lord”? But the reason is: when he heard on Sinai the section dealing with the festivals he told it to them then and he used to exhort them again about the festivals at the time when they had to be observed (lit., at the time of action) (Sifrei Bamidbar 66).