Musnad Ahmad 1313:9

Muhammad ibn Abi ‘Adi narrated to us, from Sulayman, from Abu ‘Uthman, 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." Sulayman said: He narrated it to us—meaning Abu ‘Uthman—repeatedly, and he attributed it once to the Messenger of Allah, may Allah bless him and grant him peace.