Musnad Ahmad 34:2

Yahya ibn Sa’id narrated to us, he said: al-Taymi—meaning Sulayman—narrated to us, from Abu ‘Uthman—meaning al-Nahdi—from ‘Amir ibn Malik, from Safwan ibn Umayyah, who said: "The plague, and the abdominal disease, and drowning, and the woman who dies in childbirth are martyrdom." Abu ‘Uthman narrated it to us repeatedly, and he had attributed it to the Prophet, may Allah bless him and grant him peace, once.