Five are martyrs

1247. Narrated Aboo Huraira (radhi Allaahu anhu) : Allaah’s Messenger (Sallallaahu Alaihi Wasallam) said,

“While a man was going on a way, he saw a thorny branch and removed it from the way and Allaah (Subhaanahu wata’alaa) became pleased by his action and forgave him for that.”

Then [the Prophet (Sallallaahu Alaihi Wasallam)] said,
“Five are martyrs :
One who dies of plague,
one who dies of an abdominal disease,
one who dies of drowning,
one who is buried alive (and) dies and
one who is killed in Allaah’s cause.”

[Saheeh Al-Bukhaari, Hadeeth No. 624, Vol. 1]

Source : The Book Of Al-Imaara – Chapter 51 – About the martyrs.
Al-Lulu Wal-Marjan,Compiled by : Fuwad Abdul-Baqi,Translated by : Dr. Muhammad Muhsin Khan