Musnad Ahmad 1313:8

Yazid ibn Harun narrated to us, Sulayman al-Taymi narrated to us, from Abu Uthman—meaning al-Nahdi—from Amir ibn Malik, from Safwan ibn Umayyah, from the Prophet, peace and blessings of Allah be upon him, who said: "The plague is martyrdom, drowning is martyrdom, [dying of] a stomach ailment is martyrdom, and the woman who dies in childbirth is martyrdom."