Sahih al-Bukhari 7216
Narrated Jabir:
A bedouin came to the Prophet (ﷺ) and said, "Please take my Pledge of allegiance for Islam." So the 
Prophet took from him the Pledge of allegiance for Islam. He came the next day with a fever and said 
to the Prophet (ﷺ) "Cancel my pledge." But the Prophet (ﷺ) refused and when the bedouin went away, the 
Prophet said, "Medina is like a pair of bellows (furnace): It expels its impurities and brightens and 
clears its good."
حَدَّثَنَا أَبُو نُعَيْمٍ، حَدَّثَنَا سُفْيَانُ، عَنْ مُحَمَّدِ بْنِ الْمُنْكَدِرِ، سَمِعْتُ جَابِرًا، قَالَ جَاءَ أَعْرَابِيٌّ إِلَى النَّبِيِّ صلى الله عليه وسلم فَقَالَ بَايِعْنِي عَلَى الإِسْلاَمِ‏.‏ فَبَايَعَهُ عَلَى الإِسْلاَمِ، ثُمَّ جَاءَ الْغَدَ مَحْمُومًا فَقَالَ أَقِلْنِي‏.‏ فَأَبَى، فَلَمَّا وَلَّى قَالَ ‏
"‏ الْمَدِينَةُ كَالْكِيرِ، تَنْفِي خَبَثَهَا، وَيَنْصَعُ طِيبُهَا ‏"‏‏.‏