From Sahih Muslim (Hadith 4941):
It has been narrated on the authority of Abu Huraira (through another chain of transmitters) that the Messenger of Allah (ﷺ) said:Whom do you consider to be a martyr among you? They (the Companions) said: Messenger, of Allah, one who is slain in the way of Allah is a martyr. He said: Then (if this is the definition of a martyr) the martyrs of my Umma will be small in number. They asked: Messenger of Allah, who are they? He said: One who is slain in the way of Allah is a martyr; one who dies in the way of Allah, is a martyr; one who dies of plague is a martyr; one who dies of cholera is a martyr. Ibn Miqsam said: I testify the truth of your father's statement (with regard to this tradition) that the Prophet (ﷺ) said: One who is drowned is a martyr
Reference: Muslim 4941