Musnad Ahmad 10:12

Aswad ibn Amir narrated to us, Hammad ibn Salamah narrated to us, from Muhammad ibn Ishaq, from Yazid ibn Abdullah ibn Qusayt, from Abu Salamah ibn Abd al-Rahman ibn Awf, from Abd al-Rahman ibn Awf, that a group of Arabs came to the Messenger of Allah, may Allah bless him and grant him peace, in Medina and accepted Islam. An epidemic of Medina’s fever afflicted them, so they regressed and left Medina. A group of his companions—meaning the companions of the Prophet, may Allah bless him and grant him peace—met them and said to them, "What is the matter with you that you have returned?" They said, "The epidemic of Medina afflicted us, and we found the climate of Medina unsuitable." They said, "Do you not have an example in the Messenger of Allah?" Some of them said, "They have become hypocrites," and others said, "They have not become hypocrites; they are Muslims." Then Allah, the Mighty and Majestic, revealed: "So what is the matter with you that you have become two groups regarding the hypocrites, while Allah has made them regress for what they earned?" [An-Nisa: 88].