"You have seen what I did to Egypt": I am not speaking to you from tradition. They (i.e., their punishment) is not written down for you. I am not sending you missives. I am not producing witnesses. But you yourselves have seen what they were guilty of — idolatry, illicit relations, and the spilling of blood. In the past, I did not exact punishment of them. (I did so) only for your sakes.
You Have Seen What I Did to Egypt I Am Not Speaking to
Mekhilta Tractate Bachodesh 2:4