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 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] - [صحيح مسلم: 2586]

An-Nu‘mān ibn Bashīr (may Allah be pleased with him) reported that the Messenger of Allah (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 with sleeplessness and fever."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Muslim - 2586]

Explanation

The Prophet (may Allah's peace and blessings be upon him) clarified how Muslims should act together. They should love goodness for each other, show mercy, offer help, and support each other. Moreover, they should feel hurt by whatever harm afflicts them. In other words, they should act like a single body; whenever one of its organs gets sick, the whole body responds with sleeplessness and fever.