Hawzah News Agency – The commander of the faithful, Imam Ali peace be upon him says:

إنّ الخَيرَ والشّرَّ لا يُعْرَفانِ إلّا بالنّاسِ، فإذا أرَدتَ أنْ تَعرِفَ الخَيرَ فَاعْمَلِ الخَيرَ تَعرِفْ أهلَهُ، و إذا أرَدتَ أنْ تَعرِفَ الشَّرَّ فاعْمَلِ الشَّرَّ تَعرِفْ أهلَهُ.

'Verily good and evil can only be known through people, so if you want to know good then do good and you will know its people, and if you want to know evil then commit evil and you will know its people.'

Source: Bihar al-Anwar, v. 78, p. 41, no. 26