Sahih al-Bukhari, Book 93, Hadith 69 (In-Book) ⓘ

Narrated Jabir bin `Abdullah: A bedouin gave the Pledge of allegiance to Allah's Messenger for Islam and the bedouin got a fever where upon he said to the Prophet "Cancel my Pledge." But the Prophet refused. He came to him (again) saying, "Cancel my Pledge.' But the Prophet 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."

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، عَنْ مَالِكٍ، عَنْ مُحَمَّدِ بْنِ الْمُنْكَدِرِ، عَنْ جَابِرِ بْنِ عَبْدِ اللَّهِ ـ رضى الله عنهما ـ أَنَّ أَعْرَابِيًّا بَايَعَ رَسُولَ اللَّهِ صلى الله عليه وسلم عَلَى الإِسْلاَمِ، فَأَصَابَهُ وَعْكٌ فَقَالَ أَقِلْنِي بَيْعَتِي‏.‏ فَأَبَى، ثُمَّ جَاءَهُ فَقَالَ أَقِلْنِي بَيْعَتِي‏.‏ فَأَبَى، فَخَرَجَ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ الْمَدِينَةُ كَالْكِيرِ، تَنْفِي خَبَثَهَا، وَيَنْصَعُ طِيبُهَا ‏" ‏‏.‏