Sunan an-Nasa'i 4185
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 صلى الله عليه وسلم عَلَى الإِسْلاَمِ فَأَصَابَ الأَعْرَابِيَّ وَعَكٌ بِالْمَدِينَةِ فَجَاءَ الأَعْرَابِيُّ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَ يَا رَسُولَ اللَّهِ أَقِلْنِي بَيْعَتِي ‏.‏ فَأَبَى ثُمَّ جَاءَهُ فَقَالَ أَقِلْنِي بَيْعَتِي ‏.‏ فَأَبَى فَخَرَجَ الأَعْرَابِيُّ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏
"‏ إِنَّمَا الْمَدِينَةُ كَالْكِيرِ تَنْفِي خَبَثَهَا وَتَنْصَعُ طَيِّبَهَا ‏"‏ ‏.‏