سنن ابن ماجه

It was narrated from Abu Hurairah that the Prophet (ﷺ) said: “What do you say among yourselves about the martyr?” They said: “The one who is killed in the cause of Allah.” He said: “In that case the martyrs among my nation would be few. Whoever is killed in the cause of Allah is a martyr; whoever dies in the cause of Allah is a martyr; whoever dies of a stomach disease is a martyr; and whoever dies of the plague is a martyr.”

Another chain narrates with the addition of "and the drowned is a martyr."