સાહામાનું મન ચોર, તો આપણું મન ચોર.

sāhāmānuṁ mana cōra, tō āpaṇuṁ mana cōra.

Literally: If the opponent's mind is a thief, then our mind is a thief.
What it means: One is forced to be suspicious and guarded when dealing with an untrustworthy person.

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.