Abu Hurairah says that I asked Sayyiduna Hasan bin Ali (may Allah be pleased with them both): What have you memorized from the Messenger of Allah (peace and blessings be upon him)? He said: I have memorized this (hadith) from him: "Truthfulness is tranquility and lying is doubt."
Explanation: In Tirmidhi (2518), this narration is as follows: Abu Hurairah Sa'di says that I asked Sayyiduna Hasan bin Ali radi Allahu anhu: "What have you memorized from the Messenger of Allah sallallahu alayhi wa sallam?" He said: I have memorized from the Messenger of Allah sallallahu alayhi wa sallam (this hadith): "Leave that which puts you in doubt for that which does not put you in doubt, for truth is tranquility and falsehood is doubt."
The meaning is that when your soul becomes doubtful or suspicious about something, then leave that thing, because the heart of a believer is naturally at ease with truthful actions and truthful speech, and is hesitant regarding falsehood and doubtful matters. Therefore, having doubt about something is evidence of its invalidity, so avoid it; and being at ease regarding something is evidence of its truth, so adhere to it. However, if the matter is the opposite—that one feels at ease with falsehood, is pleased with sin, and the heart is uneasy with good deeds—then this is not the voice of the heart, but rather the mischief of the commanding soul (nafs al-ammara). If the soul dominates the heart, it causes much distress, and if the heart dominates the soul, then tranquility and peace are attained.