A bedouin gave the Pledge of allegiance to Messenger of Allah ﷺ for Islam and the bedouin got a fever where upon he said to the Prophet ﷺ "Cancel my Pledge." But the Prophet ﷺ refused. He came to him (again) saying, "Cancel my Pledge.' But the Prophet ﷺ refused. Then (the bedouin) left (Medina). Allah's Apostle said: "Medina is like a pair of bellows (furnace): It expels its impurities and brightens and clears its good."  

أَنَّ أَعْرَابِيًّا بَايَعَ رَسُولَ اللَّهِ ﷺ عَلَى الإِسْلاَمِ فَأَصَابَهُ وَعْكٌ فَقَالَ أَقِلْنِي بَيْعَتِي فَأَبَى ثُمَّ جَاءَهُ فَقَالَ أَقِلْنِي بَيْعَتِي فَأَبَى فَخَرَجَ فَقَالَ رَسُولُ اللَّهِ ﷺ الْمَدِينَةُ كَالْكِيرِ تَنْفِي خَبَثَهَا وَيَنْصَعُ طِيبُهَا