Musnad Ahmad 32:1361

Ibrahim ibn Ishaq al-Taliqani narrated to us, Abdullah—meaning Ibn al-Mubarak—narrated to us, from Asim ibn Sulayman, from Hafsa bint Sirin, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The plague is martyrdom for every Muslim."