Hadith: The believers, in their mutual love, compassion, and sympathy are like a single body; if one of its organs suffers, the whole body will respond to it with sleeplessness and fever

عَنِ النُّعْمَانِ بْنِ بَشِيرٍ رضي الله عنه قَالَ: قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ:

«مَثَلُ الْمُؤْمِنِينَ فِي تَوَادِّهِمْ وَتَرَاحُمِهِمْ وَتَعَاطُفِهِمْ مَثَلُ الْجَسَدِ، إِذَا اشْتَكَى مِنْهُ عُضْوٌ تَدَاعَى لَهُ سَائِرُ الْجَسَدِ بِالسَّهَرِ وَالْحُمَّى».
[صحيح] - [متفق عليه]

An-Nu‘mān ibn Bashīr (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "The believers, in their mutual love, compassion, and sympathy are like a single body; if one of its organs suffers, the whole body will respond to it with sleeplessness and fever."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

The way the believers show mercy to one another, and communicate, and cooperate is similar to the relationship of a body with all its organs. If one organ feels pain, it will call upon the other organs to share the pain, resulting in sleeplessness and fever.