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 bin ‘Abu ‘Amra. I said: (He died) of plague. He said: The Messenger of Allah (ﷺ) said that death by plague is martyrdom for a Muslim.
حَدَّثَنَا حَامِدُ بْنُ عُمَرَ الْبَكْرَاوِيُّ، حَدَّثَنَا عَبْدُ الْوَاحِدِ، – يَعْنِي ابْنَ زِيَادٍ – حَدَّثَنَا عَاصِمٌ، عَنْ حَفْصَةَ بِنْتِ سِيرِينَ، قَالَتْ قَالَ لِي أَنَسُ بْنُ مَالِكٍ بِمَا مَاتَ يَحْيَى بْنُ أَبِي عَمْرَةَ قَالَتْ قُلْتُ بِالطَّاعُونِ . قَالَتْ فَقَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم “ الطَّاعُونُ شَهَادَةٌ لِكُلِّ مُسْلِمٍ ” .
Reference : Sahih Muslim 1916a
In-book reference : Book 33, Hadith 239
USC-MSA web (English) reference : Book 20, Hadith 4709