Sahih Bukhari Volume 8, Book 73, Hadith Number 36.

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.”)