The Forward writer Philologos in his most recent column claimed that the earliest uses of the expression והמבין יבין are found in the Ra'avad and in the Radak. Based on the fact that both these individuals lived in (late 12th century) Provence, he claims that the origin of the phrase is a Provencal translation of the New Testament. However, this expression is found earlier in the Lekach Tov (11th century Greece) and in the Ibn Ezra (early 12th c. Spain) as well as in many twelfth c. Ashkenazi works such as Rabbenu Tam's Sefer ha-Yashar, R. Yehuda b. Kalonymous' Tannaim ve-Amoraim, the Chizkuni al ha-Torah, and other places. This being the case, what is the most likely origin of the expression?
Perhaps this is the source - a take off from Mishlei 19:25