Messenger of Allah ﷺ said, "I was ordered to migrate to a town which will swallow (conquer) other towns and is called Yathrib and that is Medina, and it turns out (bad) persons as a furnace removes the impurities of iron.  

سَمِعْتُ أَبَا هُرَيْرَةَ ؓ يَقُولُ قَالَ رَسُولُ اللَّهِ ﷺ أُمِرْتُ بِقَرْيَةٍ تَأْكُلُ الْقُرَى يَقُولُونَ يَثْرِبُ وَهْىَ الْمَدِينَةُ تَنْفِي النَّاسَ كَمَا يَنْفِي الْكِيرُ خَبَثَ الْحَدِيدِ  

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.  

أَنَّ أَعْرَابِيًّا بَايَعَ رَسُولَ اللَّهِ ﷺ عَلَى الإِسْلاَمِ فَأَصَابَ الأَعْرَابِيَّ وَعْكٌ بِالْمَدِينَةِ فَأَتَى الأَعْرَابِيُّ إِلَى رَسُولِ اللَّهِ ﷺ فَقَالَ يَا رَسُولَ اللَّهِ أَقِلْنِي بَيْعَتِي فَأَبَى رَسُولُ اللَّهِ ﷺ ثُمَّ جَاءَهُ فَقَالَ أَقِلْنِي بَيْعَتِي فَأَبَى ثُمَّ جَاءَهُ فَقَالَ أَقِلْنِي بَيْعَتِي فَأَبَى فَخَرَجَ الأَعْرَابِيُّ فَقَالَ رَسُولُ اللَّهِ ﷺ إِنَّمَا الْمَدِينَةُ كَالْكِيرِ تَنْفِي خَبَثَهَا وَيَنْصَعُ طِيبُهَا  

Some of the companions of the Prophet ﷺ returned from the battle of Uhud (i.e. refused to fight) whereupon the Muslims got divided into two parties; one of them was in favor of their execution and the other was not in favour of it. So there ware revealed: "Then what is the matter with you that you are divided into two parties about the hypocrites?" (4.88). Then the Prophet ﷺ said "It (i.e. Medina) is a Taybah (good), it expels impurities as the fire expels the impurities of silver."  

رَجَعَ نَاسٌ مِنْ أَصْحَابِ النَّبِيِّ ﷺ مِنْ أُحُدٍ وَكَانَ النَّاسُ فِيهِمْ فِرْقَتَيْنِ فَرِيقٌ يَقُولُ اقْتُلْهُمْ وَفَرِيقٌ يَقُولُ لاَ۔ فَنَزَلَتْ {فَمَا لَكُمْ فِي الْمُنَافِقِينَ فِئَتَيْنِ} وَقَالَ: «إِنَّهَا طَيْبَةُ: تَنْفِي الْخَبَثَ كَمَا تَنْفِي النَّارُ خَبَثَ الْفِضَّةِ۔»