Sahih Bukhari Volume 001, Book 002, Hadith Number 035.
Sahih Bukhari Book 02. Belief
Narated By Abu Huraira : The Prophet said, "The person who
participates in (Holy battles) in Allah's cause and nothing
compels him to do so except belief in Allah and His Apostles,
will be recompensed by Allah either with a reward, or booty
(if he survives) or will be admitted to Paradise (if he is killed in
the battle as a martyr). Had I not found it difficult for my
followers, then I would not remain behind any sariya going for
Jihad and I would have loved to be martyred in Allah's cause
and then made alive, and then martyred and then made alive,
and then again martyred in His cause."