Arbaeen An-Nawawi - Hadith 6

An-Nu’man ibn Basheer R.A. reported: The Messenger of Allah, peace and blessings be upon him, said:

الْحَلَالُ بَيِّنٌ وَالْحَرَامُ بَيِّنٌ وَبَيْنَهُمَا مُشَبَّهَاتٌ لَا يَعْلَمُهَا كَثِيرٌ مِنْ النَّاسِ فَمَنْ اتَّقَى الْمُشَبَّهَاتِ اسْتَبْرَأَ لِدِينِهِ وَعِرْضِهِ وَمَنْ وَقَعَ فِي الشُّبُهَاتِ كَرَاعٍ يَرْعَى حَوْلَ الْحِمَى يُوشِكُ أَنْ يُوَاقِعَهُ أَلَا وَإِنَّ لِكُلِّ مَلِكٍ حِمًى أَلَا إِنَّ حِمَى اللَّهِ فِي أَرْضِهِ مَحَارِمُهُ أَلَا وَإِنَّ فِي الْجَسَدِ مُضْغَةً إِذَا صَلَحَتْ صَلَحَ الْجَسَدُ كُلُّهُ وَإِذَا فَسَدَتْ فَسَدَ الْجَسَدُ كُلُّهُ أَلَا وَهِيَ الْقَلْبُ​​

Verily, the lawful is clear and the unlawful is clear, and between the two of them are doubtful matters about which many people do not know. Thus, he who avoids doubtful matters clears himself in regard to his religion and his honor, and he who falls into doubtful matters will fall into the unlawful as the shepherd who pastures near a sanctuary, all but grazing therein. Verily, every king has a sanctum and the sanctum of Allah is his prohibitions. Verily, in the body there is a piece of flesh which if upright then the entire body is upright, and if corrupt then the entire body is corrupt. No doubt it is the heart.
Source: Sahih Bukhari 52, Grade: Muttafaqun Alayhi

In another narration, the Prophet said:

اجْعَلُوا بَيْنَكُمْ وَبَيْنَ الْحَرَامِ سُتْرَةً مِنَ الْحَلالِ مَنْ فَعَلَ ذَلِكَ اسْتَبْرَأَ لِعِرْضِهِ وَدِينِهِ وَمَنْ أَرْتَعَ فِيهِ كَانَ كَالْمُرْتِعِ إِلَى جَنْبِ الْحِمَى يُوشِكُ أَنْ يَقَعَ فِيهِ وَإِنَّ لِكُلِّ مَلِكٍ حِمًى وَإِنَّ حِمَى اللَّهِ فِي الأَرْضِ مَحَارِمُهُ

Make between yourself and the unlawful a barrier of what is lawful. Whoever does so will clear himself in regard to his honor and his religion. A shepherd who grazes near the side of a sanctum will nearly fall into it. Verily, every king has a sanctum and the sanctum of Allah is his prohibitions.

Source: Sahih Ibn Hibban 5685, Grade: Sahih