Parashas Bo · Shemos 12:7 · TT note 3
Torah Temimah on Shemos 12:7, note 3 (on וְעַל־הַמַּשְׁקוֹף) points to Mechilta.
Sefaria — Exodus 12:7
Sefaria — Torah Temimah on Exodus 12:7
Sefaria — Mechilta
English: “And on the lintel” — I might think on the lintel from the outside; therefore it says, “And the blood shall be for you a sign” — a sign for you and not a sign for others; hence it is only on the lintel from the inside.
Derash from לָכֶם לְאוֹת: the placement is inward-facing, for those inside the house.
The blood on the lintel (and doorposts) is applied on the inside, as a sign for Yisrael, not for outsiders.