Indeed, Allah has purchased from the believers their lives and their wealth

 "Indeed, Allah has purchased from the believers their lives and their wealth in exchange for Paradise. They fight in the cause of Allah, so they kill and are killed. [It is] a true promise [binding] upon Him in the Torah, the Gospel, and the Quran. And who is more faithful to his covenant than Allah? So rejoice in your transaction which you have contracted. And that is the great attainment."  (At-Tawbah:111)