The Prophet Muhammad (peace be upon him) said: "God will not show mercy to a person who does not show mercy to other people."
Sahih Al-Bukhari, Volume 20, Hadith 375
The Prophet also said: "(God) has mercy on those who are merciful. If you show mercy to those who are on the earth, He Who is in heaven will show mercy to you."
Sunan of Abu-Dawood, Hadith 2322