"I heard the Messenger of Allah [SAW] say: 'That which is lawful is clear and that which is unlawful is clear, but between them there are matters which are doubtful.'" And sometimes he said: "But between them are matters that are not as clear. I will describe the likeness of that for you. Allah, the Mighty and Sublime, has a sanctuary and the sanctuary of Allah is that which He has forbidden. Whoever grazes around the sanctuary will soon transgress into the sanctuary. And whoever approaches a matter that is unclear, he will soon wind up in the sanctuary." And sometimes he said: "He will soon transgress, and indeed whoever mixes in doubt, he will soon cross into it." (Using translation from Nasāʾī 5710)  

سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ «إِنَّ الْحَلَالَ بَيِّنٌ وَإِنَّ الْحَرَامَ بَيِّنٌ وَإِنَّ بَيْنَ ذَلِكَ أُمُورًا مُشْتَبِهَاتٍ» وَرُبَّمَا قَالَ «وَإِنَّ بَيْنَ ذَلِكَ أُمُورًا مُشْتَبِهَةً وَسَأَضْرِبُ فِي ذَلِكَ مَثَلًا إِنَّ اللهَ حِمًى حِمًى وَإِنَّ حِمَى اللهِ مَا حَرَّمَ وَإِنَّهُ مَنْ يَرْعَى حَوْلَ الْحِمَى يُوشِكُ أَنْ يُخَالِطَ الْحِمَى» وَرُبَّمَا قَالَ «يُوشِكُ أَنْ يَرْتَعَ وَإِنَّهُ مَنْ يُخَالِطِ الرِّيبَةَ يُوشِكُ أَنْ يَجْسُرَ»  

I said to al-Ḥasan b. ʿAlī: What did you memorize from the Messenger of Allah ﷺ? He said: I memorized from him: "Leave what makes you doubt for what does not make you doubt."  

قُلْتُ لِلْحَسَنِ بْنِ عَلِيٍّ مَا حَفِظْتَ مِنْ رَسُولِ اللهِ ﷺ؟ قَالَ: حَفِظْتُ مِنْهُ «دَعْ مَا يَرِيبُكَ إِلَى مَا لَا يَرِيبُكَ۔»