Shaykhul-Islaam Ibn al-Qayyim (may Allah have mercy upon him) stated:

When you want use something as a proof as to what is in the heart, use the movements of the tongue as a proof. For indeed it makes known that which is in the heart whether the person wills it to be that way or refuses.

Source: Ad-Daa’-u wad-Dawaa’, pg. 64

Translated by Abu Yusuf Khaleefah
16th of Rabee’ ath-Thaani, 1436 H (02-05-2015)
Masjid Nur Allah, Queens, NYC

Original Arabic

:‏قال الإمام ابن القيم

وإذا أردت أن تستدل على ما في القلب ، فاستدل عليه بحركة اللسان ، فإنه يُطلِع ما في القلب ، شاء صاحبه أم أبى

 الداء والدواء ص٣٦٤