The Prophet(S.A.W) said:
The relationship of the believer with another believer is like (the bricks of) a building, each strengthens the other. He (S.A.W) illustrated this by interlacing the fingers of both his hands.
[Bukhari : 2266]

This Hadith enjoins unity in the Muslim community, each member of which is like a brick, which, when they are all combined, lend strength to each other. Similarly, Muslims are like hands and arms, which physically joined together, are bound to co-operate with one another.