Sahih Bukhari Volume 1, Book 10, Hadith 653

ثُمَّ قَالَ ‏"‏ الشُّهَدَاءُ خَمْسَةٌ الْمَطْعُونُ، وَالْمَبْطُونُ، وَالْغَرِيقُ، وَصَاحِبُ الْهَدْمِ، وَالشَّهِيدُ فِي سَبِيلِ اللَّهِ ‏"‏‏.‏ وَقَالَ ‏"‏ لَوْ يَعْلَمُ النَّاسُ مَا فِي النِّدَاءِ وَالصَّفِّ الأَوَّلِ ثُمَّ لَمْ يَجِدُوا إِلاَّ أَنْ يَسْتَهِمُوا لاَسْتَهَمُوا عَلَيْهِ ‏"

Then the Prophet 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 Allah's cause." (The Prophet further said, "If the people knew the reward for pronouncing the Adhan and for standing in the first row (in the congregational prayer) and found no other way to get it except by drawing lots they would do so,