A bedouin gave the Pledge of allegiance to Messenger of Allah ﷺ for Islam. Then the bedouin got fever at Medina, came to Messenger of Allah ﷺ and said, "O Messenger of Allah ﷺ! Cancel my Pledge," But Allah's Apostle refused. Then he came to him (again) and said, "O Messenger of Allah ﷺ! Cancel my Pledge." But the Prophet ﷺ refused Then he came to him (again) and said, "O Messenger of Allah ﷺ! Cancel my Pledge." But the Prophet ﷺ refused. The bedouin finally went out (of Medina) whereupon Messenger of Allah ﷺ said, "Medina is like a pair of bellows (furnace): It expels its impurities and brightens and clears its good. (Using translation from Bukhārī 7211)  

أَنَّ أَعْرَابِيًّا بَايَعَ رَسُولَ اللهِ ﷺ عَلَى الْإِسْلَامِ فَأَصَابَ الْأَعْرَابِيَّ وَعْكٌ بِالْمَدِينَةِ فَجَاءَ الْأَعْرَابِيُّ إِلَى رَسُولِ اللهِ ﷺ فَقَالَ يَا رَسُولَ اللهِ أَقِلْنِي بَيْعَتِي فَأَبَى ثُمَّ جَاءَهُ فَقَالَ أَقِلْنِي بَيْعَتِي فَأَبَى رَسُولُ اللهِ ﷺ فَخَرَجَ الْأَعْرَابِيُّ فَقَالَ رَسُولُ اللهِ ﷺ «إِنَّمَا الْمَدِينَةُ كَالْكِيرِ تَنْفِي خَبَثَهَا وَيَنْصَعُ طِيبُهَا»