This is part of Hadith #6 of Imam Nawawi’s famous collection of 40 Hadith. The Hadith itself is longer and speaks about taking caution to avoid doubtful matters. In this part, our Prophet (ﷺ) informs us of the importance of rectifying our hearts. See link below for full lesson with our brother Abu Arwa Ali.
Full Hadith:
From Abū ʿAbd Allāh al-Nuʿmān bin Bashīr (raḍiyallāhu ʿanhumā) who said:
I heard the Messenger of Allāh (ṣallallāhu alayhi wasallam) saying, “That which is lawful is clear and that which is unlawful is clear and between the two are ambiguous matters which many people do not know of. So whoever took caution regarding the ambiguous matters has absolved himself regarding his religion and his honour. And whoever fell into the doubtful matters has fallen into the unlawful, just like the shepherd who grazes his flock around the sanctuary (of a king), all but grazing therein. Indeed, every king has a sanctuary (that cannot be violated) and indeed the sanctuary of Allāh is his prohibitions. Indeed, there is a morsel of flesh in the body which if it is sound the whole body is sound and which if itis corrupt the whole body is corrupt. Indeed, it is the heart.”
[Related by al-Bukhārī, Muslim]