Physically, human beings are tied together by a common drop of Adam's blood. But the psychological and spiritual bonds of faith tie them tighter together.
Du'a -- praying for each other -- is a rope that God dropped from heaven to bind us together on earth.
Across all the earthly distances that separate human beings in this world, Du'a, this rope of Allah, creates bonds of caring and sharing their joys and sorrows among them.
Believers then become like one single body -- when one part suffers, the entire body responds with fever and aches and wakefulness.
And that is a Hadith of Prophet Muhammad, Sallallahu Alaihi wa Sallam.