Sahih Bukhari Volume 008, Book 073, Hadith Number 036.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 73. Good Manners and Form (Al-Adab)
Narated By Abu Huraira : Allah's Apostle said, "The one who looks after and works for a widow and for a poor person is like a warrior fighting for Allah's Cause." (The narrator Al-Qa'nabi is not sure whether he also said "Like the one who prays all the night without slackness and fasts continuously and never breaks his fast.")