The Messenger of Allah ﷺ said, "Jibreel came to me with the fever and the plague. I kept the fever in Madinah and sent the plague to Sham. The plague is martyrdom and mercy for my Ummah, and a torment upon the disbeliever."
يقول: قال رسول الله ﷺ: أتاني جبريل بالحمى، والطاعون، فأمسكت الحمى بالمدينة، وأرسلت الطاعون إلى الشام، فالطاعون شهادة لأمتي، ورحمة، ورجس على الكافر.
Ahmad (20767), Ibn Abu Asim in Al-Ahad wal-Mathani (466), and Tabarani in Al-Kabir (22/391) all narrated it, all through the route of Yazid bin Harun, who said, Muslim bin Ubayd Abu Nusayrah narrated to us, saying, I heard Abu Usayb, the freed slave of the Messenger of Allah ﷺ, say, and he then mentioned it.Its chain is hasan because of Muslim bin Ubayd, for his hadith is hasan. Haythami said in Al-Majma (3102), "Ahmad and Tabarani narrated it in Al-Kabir, and the men of Ahmad's chain are trustworthy."As for his saying, "So I kept the fever in Madinah and sent the plague to Sham," the wisdom in this is that when he ﷺ entered Madinah, his companions were few in number and in support, and Madinah was plague-ridden. So the Prophet ﷺ was given a choice between two things, and he chose fever at that time, since death from it was generally rare, unlike the plague. Then, when he needed to fight the disbelievers and saw that the fever was weakening their bodies, he prayed for the fever to be moved from Madinah to Juhfah.See Fath al-Bari by Ibn Hajar (10/191).