Why we should make dua for one another

السلام عليكم

بِسْمِ اللهِ الرَّحْمنِ الرَّحِيمِ

A Muslim's supplication in the absence of his brother is accepted. An angel is stationed at his head; whenever he makes supplication for the good of his brother, this angel says: Amin and may you receive the same.

[Muslim]