Imam al-Shafi'i (may Allah have mercy on him) said: "Because being killed is martyrdom for the Muslims, and Islam is far too honourable for a polytheist to be given (wealth) on the condition that he refrain from (fighting) the people of Islam, because the people of Islam, whether they kill or are killed, are (in every case) prevailing upon the truth."
The Shaykh (Imam al-Bayhaqi) (may Allah have mercy on him) said: "It has reached us in the hadith of Sayyiduna al-Mughirah bin Shu'bah (may Allah be pleased with him) about the episode of al-Ahwaz that he said: 'Our Prophet (peace and blessings be upon him) informed us of the message of our Lord that whoever of us is killed will go to a Paradise and blessings the like of which have never been seen, and whoever of us remains will become the master of your necks.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / Q18822