Leave off that which does not concern you

بـسـم الله والحـمـد لله والـصلاة والـسـلام عــلى رسـول الله، وبـعـد

On the authority of Abu Hurairah: The Messenger of Allaah صلى الله عليه وسلم said: “Part of someone's being a good Muslim is his leaving alone that which does not concern him.” [Tirmidhee (2317) and Ibn Majah (3976) and Authenticated by Shaikh al-Albaanee]

A question was asked to Shaikh al-`Uthaymeen:

Question: Does it mean that when a person leaves off that which does not concern him that he leaves off enjoining what is good and forbidding what is evil?

Answer: No! Because enjoining what is good and forbidding what is evil is from the things which concerns a person. As Allaah تعالى said:

{Let there arise out of you a group of people inviting to all that is good (Islaam), enjoining Al-Ma`roof (i.e. Islamic Monotheism and all that Islam orders one to do) and forbidding Al-Munkar (polytheism and disbelief and all that Islam has forbidden)} [Surah aal-`Imraan (3):104] 

So if a person sees another man doing munkar (evil) and he says to him, “Oh my brother, this is munkar and it is not allowed.” So it is not upon the person to reply back to him, “This does not concern you.” And even if he says that, he (the first person) should not accept it from him because enjoining what is good and forbidding what is evil concerns the entire Muslim Ummah. 

And from it is also that which concerns the wives, the sons and the daughters; for he is like the shepherd (guardian) of the house. He explains to them what is good and commands them to it. And he warns them against evil and forbids them from it. As Allaah تعالى said:

{O you who believe! Ward off yourselves and your families against a Fire (Hell) whose fuel is men and stones...} [Surah al-Tahreem (66):6]

Reference:

Sharh al-Arba`een al-Nawawee by Shaikh al-`Uthaymeen