Musnad Ahmad Hadith 22136

Abu Qilabah says that when the plague spread in Syria, Amr bin Al-Aas (may Allah be pleased with him) said to the army, "This punishment has descended, so to save yourselves from it, go into the mountain passes and valleys." When Mu'adh (may Allah be pleased with him) heard this, he did not confirm his statement and said, "Rather, this is martyrdom and mercy and the supplication of your Noble Prophet (peace and blessings of Allah be upon him). O Allah! Grant Mu'adh and his family a share of Your mercy." Abu Qilabah (may Allah have mercy on him) says, "I understood the meaning of martyrdom and mercy, but I did not understand what was meant by the supplication of the Noble Prophet (peace and blessings of Allah be upon him). Later, I found out that once the Noble Prophet (peace and blessings of Allah be upon him) was praying at night, and while supplicating, he said, 'Then fever or plague,' repeating this phrase three times. In the morning, someone from his family asked, 'O Messenger of Allah! I heard you making this supplication last night?' The Noble Prophet (peace and blessings of Allah be upon him) asked, 'Did you really hear that supplication?' He said, 'Yes!' The Noble Prophet (peace and blessings of Allah be upon him) said, 'I requested my Lord not to destroy my Ummah due to famine, so my Lord accepted this supplication. Then I requested that He not impose an external enemy upon them who would shed their blood freely, so my Lord accepted this supplication as well. Then I requested that they not be divided into different sects so that they taste each other's violence, but He did not accept this request. Upon this, I said, "Then fever or plague," three times.'"