What the whole thing is compared to
The comparison is not a community. It is an organism.
تَرَى الْمُؤْمِنِينَ فِي تَرَاحُمِهِمْ وَتَوَادِّهِمْ وَتَعَاطُفِهِمْ كَمَثَلِ الْجَسَدِ
Tara-l-muʾminīna fī tarāḥumihim wa tawāddihim wa taʿāṭufihim ka-mathali-l-jasad
“You see the believers in their mercy to one another, their love for one another and their kindness to one another, like a single body.”
Where this comes from
It continues: when one limb complains, the rest of the body answers it with sleeplessness and fever. Not that the others help. That they cannot sleep.
The full narration
Narrated An-Nu`man bin Bashir:Allah's Messenger (ﷺ) said, "You see the believers as regards their being merciful among themselves and showing love among themselves and being kind, resembling one body, so that, if any part of the body is not well then the whole body shares the sleeplessness (insomnia) and fever with it
Source
Ṣaḥīḥ al-Bukhārī · 6011
- Agreed upon as authentic (al-Bukhārī / Muslim)