"And all the children of Israel did so" (Exodus 12:50). This is to give them reward for the going equal to the reward for the doing — as it is written above, "and the children of Israel went and did" (Exodus 12:28), and so on, that it was likewise in their hearts to do it.
Israel Rewarded for the Going as Much as for the Doing
Mekhilta DeRabbi Shimon Ben Yochai 12:50
ויעשו כל בני ישראל ליתן להן שכר הליכה כשכר עשייה (כי דכתיב לעיל וילכו ויעשו בני ישראל כול' עד וכן היה בלבם לעשות):