Medina is like a furnace

Narrated Jabir (Radi-Allahu 'anhu):

A bedouin came to the Prophet (Sallallahu 'Alaihi Wa Sallam) and gave a
pledge of allegiance for embracing Islam. The next day he came with fever
and said (to the Prophet), "Please cancel my pledge (of embracing Islam and
of emigrating to Medina)." The Prophet (Sallallahu 'Alaihi Wa Sallam)
refused (that request) three times and said, "Medina is like a furnace, it
expels out the impurities (bad persons) and selects the good ones and makes
them perfect."

Bukhari Vol. 3 : No. 107