It was narrated from Jabir bin 'Abdullah that:

a Bedouin pledged Islam to the Messenger of Allah, then the Bedouin was stricken with the fever in Al-Madinah. So he came to the Messenger of Allah and said: "O Messenger of Allah, cancel my pledge," but he refused. Then he came to him again and said: "Cancel my pledge," but he refused. Then he came to him again and said: "Cancel my pledge," but he refused. Then the Bedouin left (Al-Madinah) and the Messenger of Allah sadi: "Al-Madinah his like the bellows; it expels its dross and brightness its good.

أَخْبَرَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنْ مُحَمَّدِ بْنِ الْمُنْكَدِرِ، عَنْ جَابِرِ بْنِ عَبْدِ اللَّهِ، أَنَّ أَعْرَابِيًّا، بَايَعَ رَسُولَ اللَّهِ ﷺ عَلَى الإِسْلاَمِ فَأَصَابَ الأَعْرَابِيَّ وَعَكٌ بِالْمَدِينَةِ فَجَاءَ الأَعْرَابِيُّ إِلَى رَسُولِ اللَّهِ ﷺ فَقَالَ يَا رَسُولَ اللَّهِ أَقِلْنِي بَيْعَتِي . فَأَبَى ثُمَّ جَاءَهُ فَقَالَ أَقِلْنِي بَيْعَتِي . فَأَبَى فَخَرَجَ الأَعْرَابِيُّ فَقَالَ رَسُولُ اللَّهِ ﷺ " إِنَّمَا الْمَدِينَةُ كَالْكِيرِ تَنْفِي خَبَثَهَا وَتَنْصَعُ طَيِّبَهَا " .