Answer: An-Nu‘mān ibn Bashīr (may Allah be pleased with him and his father) reported that he heard the Prophet (may Allah's peace and blessings be upon him) say: "Verily, there is a piece of flesh in the body; if it is upright then the entire body will be upright, and if it is corrupt then the entire body will be corrupt. Verily, it is the heart." [Narrated by Al-Bukhāri and Muslim]

Benefits of the Hadīth

1. Uprightness of the heart ensures one's apparent and hidden uprightness.

2. Paying attention to the uprightness of the heart because a human's uprightness is dependent upon it.

Eleventh Hadīth