Surah No. 2, Al Baqr, Ayat No. 154
Surah No. 3, Al Imraan, Ayat No. 157
And if you are killed or die in the cause of Allah, forgiveness and mercy from Allah are far better than all that they could amass (of worldly wealths).
Surah No. 3, Al Imraan, Ayat No. 169
The word 'death' as well as its general concept has a depressing effect on people. Therefore people have been instructed not to refer to martyrs who laid down their lives for God as 'dead', since this might lead to a fear which enables people to struggle and make sacrifices in God's cause. In fact, whenever there was a need to go on Jihad in the path of Allah the people had a fear of death. Which might have lead to the desperation among that time of generation. Allah had made them realized to bear in mind that anyone who lays down his life for Allah has in fact attained immortality. This revelation has brought lot of courage in the hearts of the believers.