ונתתי את חן העם הזה בעיני מצרים והיה כי תלכון לא תלכו ריקם (שמות ג:כא)

It is brought down from the Midrash that the Yidden became rich whenever Hashem brought the Makkos – plagues upon the Egyptians.  It is alluded to in the passuk מצרים והי’ כי תלכון- the first letters of these four words form the word “מכות,” and the passuk finishes לא תלכו ריקם – through the makkos, you will not leave empty-handed, since you will become rich.