Is it a condition for a sound heart to experience unveilings of hidden matters, as the Sufis say that "when the heart becomes pure, it is like a mirror, seeing of the hidden what others do not see," or can a heart be sound without these unveilings occurring?

This statement is incorrect, and its falsehood is indicated by the fact that the Companions did not frequently experience unveiling (kashf), despite being the purest of people in heart, as Al-Badawi said: "The Companions of the Chosen One (Prophet Muhammad) did not yearn for miracles; rather, their aim was to attain steadfastness (istiqamah), and few among them became known for unveiling."

A sound heart is a heart that is free from every desire that contradicts Allah's command and prohibition, and from every doubt that opposes His revelation. It is free from the servitude of anything other than Him and from seeking judgment from anyone other than His Messenger. It is the heart whose servitude to Allah alone is purified in will, love, reliance, and repentance, and whose actions are purified for Allah. It does not love except for Allah's sake, and does not hate except for Allah's sake. It must also be free from submission and seeking judgment from anyone other than the Messenger. Thus, the judge over it is what came from the Messenger, peace be upon him, so it does not put forth any belief, statement, or action before him.

Righteous deeds require two things: sincerity to Allah and adherence to the Messenger, peace be upon him. These are the foundation of a sound heart, which guarantees salvation and happiness.