Is it true that all Prophets are martyrs and that they will receive the reward of martyrs in the Hereafter from Allah, the Almighty?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If you mean by martyrs that all the Prophets were killed in the cause of Allah, then that is not true. However, though most of them died on their beds, their rewards are much greater than those of the martyrs and others as they are held in the most refined status among all creation according to the unanimous agreement of people (Muslim scholars). Allah, The Exalted, gave them precedence in merit and honor in the verse (which means): {Those will be with the ones upon whom Allah has bestowed favor of the Prophets, the steadfast affirmers of truth, the martyrs and the righteous. And excellent are those as companions.} [Quran 4:69]

Al-Kalaabaathi wrote, “They (Muslim scholars) unanimously agreed that the Prophets are the best of mankind, and there is no human being who is held in the same refined status of the Prophets, neither a Wali (a very pious person), nor a Siddeeq (affirmer of truth), nor anyone else, no matter how high his status is.”

Allah knows best.