Sahih Al Bukhari The Book of Belief Hadith No 36

The Prophet (PBUH) 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.”