દેખાય છે દેડકા, પણ માંહેથી કાળા નાગ.

dēkhāya chē dēḍakā, paṇa māṁhēthī kāḷā nāga.

Literally: They look like frogs, but inside they are black cobras.
What it means: Said of people who appear harmless but are actually dangerous and malicious.

Also said as

ગુજરાતી કહેવતસંગ્રહ (Gujarati Kahevat Sangrah) (1923) ગુજરાતી કહેવતસંગ્રહ (Gujarati Kahevat Sangrah) (1921) · read from the scan by machine — the Gujarati itself is unchecked · English is machine-made and nobody has checked it yet

Heard it differently, or know we've got the sense wrong? Tell us — that is exactly the kind of correction this needs.