It was narrated that 'Abdullah bin Masood (رضي الله عنه) said:

Beware of saying. So and so died as a martyr or So and so was killed as a martyr, for a man may fight to get booty, or he may fight to be remembered, or he may fight to show off. If you must bear witness that someone is a martyr, then bear witness for the group whom The Messenger of Allah (ﷺ) sent on a campaign and they were killed, and they said: O Allah, tell our Prophet (ﷺ) about us, that we have inet You and we are pleased with You and You are pleased with us.