“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."
عَنِ النَّبِيِّ ﷺ أَنَّهُ قَالَ مَا تَقُولُونَ فِي الشَّهِيدِ فِيكُمْ قَالُوا الْقَتْلُ فِي سَبِيلِ اللَّهِ قَالَ إِنَّ شُهَدَاءَ أُمَّتِي إِذًا لَقَلِيلٌ مَنْ قُتِلَ فِي سَبِيلِ اللَّهِ فَهُوَ شَهِيدٌ وَمَنْ مَاتَ فِي سَبِيلِ اللَّهِ فَهُوَ شَهِيدٌ وَالْمَبْطُونُ شَهِيدٌ وَالْمَطْعُونُ شَهِيدٌ قَالَ سُهَيْلٌ وَأَخْبَرَنِي عُبَيْدُ اللَّهِ بْنُ مِقْسَمٍ عَنْ أَبِي صَالِحٍ وَزَادَ فِيهِ وَالْغَرِقُ شَهِيدٌ