Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
ibnhibban:297b

[Machine] And I heard the Messenger of Allah (peace be upon him) saying, "Verily, in the body there is a morsel of flesh, if it is sound, then the whole body is sound, and if it is corrupt, then the whole body is corrupt. Indeed, it is the heart."  

ابن حبّان:٢٩٧b

وَسَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ «إِنَّ فِي الْجَسَدِ مُضْغَةً إِذَا صَلَحَتْ صَلَحَ لَهَا الْجَسَدُ وَإِذَا فَسَدَتْ فَسَدَ لَهَا الْجَسَدُ كُلُّهُ»