Musar do Powtórzonego Prawa 4:37

When G–d tells Israel in 19,4: "You have seen what I have done to Egypt, etc.," this occurred at a time when they had not yet been completely purified, and were still subject to the attribute of Justice, just like the Egyptians. The attribute of Justice had wanted to know why- seeing that both the Israelites and the Egyptians worshipped idols- the former should experience redemption. The only reason G–d was able to take the Israelites out of Egypt at the time was that they were descended from צדיקים, whereas the other Egyptians were descended from רשעים. The Israelites, however, had retained a spark of their erstwhile holiness and hence the Torah said of them: ותחת כי אהב את אבותיך ויבחר בזרעו אחריו ויוציאך בפניו בכוחו הגדול ממצרים, "And because He loved your fathers, He chose their heirs after them; He Himself in His great might led you out of Egypt" (Deut. 4 37). This verse tells of the great favour G–d did for the Jewish people by taking them out of Egypt; they witnessed how G–d treated a רשע בן צדיק (in this case).