Musnad Ahmad 1313:2

Yahya ibn Sa'id narrated to us, he said: Sulayman at-Taymi narrated to us, from Abu 'Uthman an-Nahdi, from 'Amir ibn Malik, from Safwan ibn Umayyah, who said: "The plague, and the stomach ailment, and drowning, and the woman who dies in childbirth are martyrdom." He said: Abu 'Uthman narrated this to us repeatedly, and he attributed it to the Prophet, may Allah bless him and grant him peace, once.