Sahih Muslim
The Book on Government (Kitab Al-Imara)
Book Number: 020 / Hadith Number: 4709
It has been narrated on the authority of Hafsa daughter of Sirin who said: Anas b. Malik asked me the cause of death of Yahya b. 'Abu 'Amra. I said: (He died) of plague. He said: The Messenger of Allah (may peace be upon him) said that death by plague is martyrdom for a Muslim.