In the Name of Allah, the Most Gracious, the Most Merciful.
Allah says: "The believers are but brothers." (49:10)
This ayah was revealed in Madinah, after the Hijrah, when the Prophet (peace be upon him) had established the bonds of brotherhood between the Muhajirun and the Ansar. He paired each Muhajir with an Ansari, and they became brothers not by blood, but by faith.
The bond was real. The Ansar shared their wealth, their homes, and their trade with the Muhajirun. They did not regard them as guests, but as family. When the wealth of Bahrain arrived, the Prophet (peace be upon him) offered it to the Ansar, but they refused, saying: "Leave it for the Muhajirun, and we will share our wealth with them." (Bukhari)
The brotherhood extended beyond material support. The companions guarded each other's honour, made dua for one another, visited each other in sickness, and attended each other's funerals. Abu Bakr and Umar, may Allah be pleased with them, differed on the council of succession, yet their love for each other was visible to all.
This brotherhood is not a thing of the past. It is the inheritance of every believer. The Prophet (peace be upon him) said: "The simile of the believers in their mutual love, mercy, and compassion is that of a single body; when one limb suffers, the rest of the body responds with sleeplessness and fever." (Muslim)
We live in an age of fragmentation, where the bonds of faith have been replaced by the bonds of blood, nation, and interest. The ummah has paid a heavy price for this. To return to the model of the companions is to return to the source of our strength.
May Allah unite our hearts upon His obedience, and make us brothers who love each other for His sake.