Sahih Bukhari Volume 4, Book 52, Hadith Number 142.

Narated By Sahl bin Sad As-Sa’di  : Allah’s Apostle said, “To guard Muslims from infidels in Allah’s Cause for one day is better than the world and whatever is on its surface, and a place in Paradise as small as that occupied by the whip of one of you is better than the world and whatever is on its surface; and a morning’s or an evening’s journey which a slave (person) travels in Allah’s Cause is better than the world and whatever is on its surface.”