Charles Ellicott Commentary Exodus 12:13

"And the blood shall be to you for a token upon the houses where ye are: and when I see the blood, I will pass over you, and there shall no plague be upon you to destroy you, when I smite the land of Egypt." — Exodus 12:13 (ASV)

The blood shall be to you for a token. —Rather, the blood shall be for a token for you: i.e., it shall be a token to Me on your behalf. (See the comment on Exodus 12:7, and compare Exodus 12:23.)