Hadith: Verily, the lawful is clear, and the unlawful is clear

عن النُّعمان بن بَشير رضي الله عنه قال: سَمِعْتُ رَسُولَ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ يَقُولُ -وَأَهْوَى النُّعْمَانُ بِإِصْبَعَيْهِ إِلَى أُذُنَيْهِ-:
«إِنَّ الْحَلَالَ بَيِّنٌ وَإِنَّ الْحَرَامَ بَيِّنٌ، وَبَيْنَهُمَا مُشْتَبِهَاتٌ لَا يَعْلَمُهُنَّ كَثِيرٌ مِنَ النَّاسِ، فَمَنِ اتَّقَى الشُّبُهَاتِ اسْتَبْرَأَ لِدِينِهِ وَعِرْضِهِ، وَمَنْ وَقَعَ فِي الشُّبُهَاتِ وَقَعَ فِي الْحَرَامِ، كَالرَّاعِي يَرْعَى حَوْلَ الْحِمَى يُوشِكُ أَنْ يَرْتَعَ فِيهِ، أَلَا وَإِنَّ لِكُلِّ مَلِكٍ حِمًى، أَلَا وَإِنَّ حِمَى اللهِ مَحَارِمُهُ، أَلَا وَإِنَّ فِي الْجَسَدِ مُضْغَةً، إِذَا صَلَحَتْ صَلَحَ الْجَسَدُ كُلُّهُ، وَإِذَا فَسَدَتْ فَسَدَ الْجَسَدُ كُلُّهُ، أَلَا وَهِيَ الْقَلْبُ».
[صحيح] - [متفق عليه] - [صحيح مسلم: 1599]

An-Nu'mān ibn Bashīr (may Allah be pleased with him) reported: I heard the Messenger of Allah (may Allah's peace and blessings be upon him) say, as An-Nu'mān dropped his two fingers to his ears:
"Verily, the lawful is clear, and the unlawful is clear, and between them are doubtful matters many people do not know. Whoever avoids doubtful matters clears his liability regarding his religion and his honor, and whoever falls into doubtful matters will fall into the unlawful, just like the shepherd who grazes his animals in the vicinity of a prohibited pasture and is thus likely to graze therein. Verily, every king has a protected area, and the protected area of Allah is His prohibitions. Verily, in the body, there is a piece of flesh; if it is sound, the entire body will be sound, and if it is corrupt, the entire body will be corrupt; verily, that is the heart."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Muslim - 1599]

Explanation

The Prophet (may Allah's peace and blessings be upon him) explains a general rule in matters and that they fall under three categories in the Shariah: clearly lawful matters, clearly unlawful matters, and doubtful matters with the rulings on whether they are illegal or lawful being unclear and unknown to many people.
So, whoever abandons those suspicious matters keeps his religion safe by avoiding falling into the unlawful and keeps his honor safe from people's criticism of him for engaging in such doubtful things. Whoever does not avoid doubtful matters risks falling into the unlawful or being maligned by people. The Messenger (may Allah's peace and blessings be upon him) provided a parable to demonstrate the condition of a person who engages in doubtful matters, saying that he is like a shepherd who grazes his cattle close to a piece of land protected by its owner; so, his cattle are likely to pasture in this protected land, given its proximity. Likewise, he who engages in a suspicious matter draws near to the unlawful and is likely to fall into it. Then, the Prophet (may Allah's peace and blessings be upon him) informed that there is a piece of flesh in the body, namely the heart, and when it is sound, the whole body is sound, and if it is corrupt, the entire body becomes corrupt.