Rashi on Exodus 7:16/רש"י על שמות ז׳:ט״ז

1עד כה means UNTIL NOW. A Midrashic explanation (taking this sentence to mean, “thou wilt not hearken until (עד) thou hearest the word “כה”) is: until thou hearest from me the announcement of the slaughter of the first-born, which I will begin with the words, (Exodus 11:4) “Thus (כה) saith the Lord, About midnight etc.”