Matthew 5:7 reads, "Blessed are the merciful: for they shall obtain mercy."
Nephi 12:7 reads, "And blessed are the merciful, for they shall obtain mercy."
This teaching is as clear as any teaching can be; yet the practice of it can be difficult.
I won't expound on this one too much, other than to point out a recent and popular talk given in the April 2012 general conference.
The Merciful Obtain Mercy by Dieter F. Uchtdorf.