Volume 1, Book 4, Number 174:
Narrated Abu Huraira Razi Allah Anhu: The Prophet Peace Be Upon Him said, “A man saw a dog eating mud from (the severity of) thirst. So, that man took a shoe (and filled it) with water and kept on pouring the water for the dog till it quenched its thirst. So Allah approved of his deed and made him to enter Paradise.”
http://www.holyebooks.org/islam/hadith_of_bukhari/004.sbt.html