Sunan an-Nasa'i, Hadith 2054 ⓘ
It was narrated that Safwan bin Umayyah said: "The plague, abdominal illness, drowning and dying in childbirth are martyrdom." (One of the narrators) said: Abu 'Uthman narrated this to us several times, and on one occasion he attributed it to the Prophet.
أَخْبَرَنَا عُبَيْدُ اللَّهِ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا يَحْيَى، عَنِ التَّيْمِيِّ، عَنْ أَبِي عُثْمَانَ، عَنْ عَامِرِ بْنِ مَالِكٍ، عَنْ صَفْوَانَ بْنِ أُمَيَّةَ، قَالَ الطَّاعُونُ وَالْمَبْطُونُ وَالْغَرِيقُ وَالنُّفَسَاءُ شَهَادَةٌ . قَالَ وَحَدَّثَنَا أَبُو عُثْمَانَ مِرَارًا وَرَفَعَهُ مَرَّةً إِلَى النَّبِيِّ صلى الله عليه وسلم .
also referenced under: Book 21, Hadith 238 (In-Book) & Volume 3, Book 21, Hadith 2056 (DarusSalam)