There is an often-quoted saying that
things that come from the heart enter the heart,
i.e., if something is said sincerely, the listener knows, and if it is said insincerely, the listener knows that, too.
In Hebrew, it is quoted as:
דברים היוצאים מן הלב נכנסים ללב
דברים היוצאים מן הלב נכנסים אל הלב.
Many sources (ex. Kedushas Levi, Vayigash 2; Sefas Emes, Shoftim 7:3; Likutei Ma'amarim, Festivals 11:2; Noam Eliezer, Haazinu 2:1, et. al.) quote this as "Chazal say," but I have yet to find the original Ma'amar. Does anyone know where it can be found?