On the authority of Abu 'Abdullah al-Nu'man bin Bashir, radiyallahu 'anhu, who said: I heard the Messenger of Allah, sallallahu 'alayhi wasallam, say:
"Truly, what is lawful is evident, and what is unlawful is evident, and in between the two are matters which are doubtful which many people do not know. He who guards against doubtful things keeps his religion and honour blameless, and he who indulges in doubtful things indulges in fact in unlawful things, just as a shepherd who pastures his flock round a preserve will soon pasture them in it. Beware, every king has a preserve, and the things Allah has declared unlawful are His preserves. Beware, in the body there is a flesh; if it is sound, the whole body is sound, and if it is corrupt, the whole body is corrupt, and behold, it is the heart."
Lesson from this Hadith
In another related hadith the Prophet, sallallahu 'alayhi wasallam, mentioned that: "The heart will be exposed to fitan (trials) again and again. The heart which is influenced by fitnah will have a black spot placed on it. And if the heart avoids the fitnah, there will be a white spot on it." The heart then is either a pure one, or a heart that has been sealed after being affected by fitan or prohibited or doubtful matters again and again. Even though the heart pumps out blood and the person is alive, the heart in reality is not alive, because the iman is weak, making the heart unsound. Hence avoiding and getting away from the fitan is important in order to have a pure heart, especially in these contemporary times where it is full of evil that attracts the heart.
There are doubtful matters that a Muslim might face in his/her everyday dealings and activities. This hadith helps the Muslim in dealing with such matters. There is an early ongoing preparation which is needed in such a case - to purify one's heart. There are some actions that can help us achieve this task, like: hayya' (modesty), murakabah (self reckoning), muhasabah (self accountability), dzikr (remembrance of Allah), and as-Salatu 'ala an-Nabi, sallallahu 'alayhi wasallam. These are things that, insha Allah, can help us and enlighten and purify our heart, and also help us to identify and avoid the doubtful matters - this is the main point, the key, the principle of the hadith.
"Beware; in the body there is a flesh; if it is sound, the whole body is sound, and if it is corrupt, the whole body is corrupt, and behold, it is the heart."