Sahih al-Bukhari 36
Narrated Abu Huraira:
The Prophet (ﷺ) 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."
حَدَّثَنَا حَرَمِيُّ بْنُ حَفْصٍ، قَالَ حَدَّثَنَا عَبْدُ الْوَاحِدِ، قَالَ حَدَّثَنَا عُمَارَةُ، قَالَ حَدَّثَنَا أَبُو زُرْعَةَ بْنُ عَمْرِو بْنِ جَرِيرٍ، قَالَ سَمِعْتُ أَبَا هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏
"‏ انْتَدَبَ اللَّهُ لِمَنْ خَرَجَ فِي سَبِيلِهِ لاَ يُخْرِجُهُ إِلاَّ إِيمَانٌ بِي وَتَصْدِيقٌ بِرُسُلِي أَنْ أُرْجِعَهُ بِمَا نَالَ مِنْ أَجْرٍ أَوْ غَنِيمَةٍ، أَوْ أُدْخِلَهُ الْجَنَّةَ، وَلَوْلاَ أَنْ أَشُقَّ عَلَى أُمَّتِي مَا قَعَدْتُ خَلْفَ سَرِيَّةٍ، وَلَوَدِدْتُ أَنِّي أُقْتَلُ فِي سَبِيلِ اللَّهِ ثُمَّ أُحْيَا، ثُمَّ أُقْتَلُ ثُمَّ أُحْيَا، ثُمَّ أُقْتَلُ ‏"‏‏.‏