From The Excellence of A Persons Islaam

Leaving Alone That in Which Doesn't Concern You Taken from the explanation of Bulughul Maraam by as-Shaykh Saalih al-Fawzaan حفظه الله 

On the Authority of Abu Hurairah رضي الله عنه who said that the messenger of Allaah (May the peace and blessings of Allaah be upon him) said: 

“From the excellence of a persons Islam is his leaving that which does not concern him” (collected by at-Tirmidhi who said hadeeth Hasan) 

Explanation: 
This is also from the four ahaadeth, 
that in which the deen is revolved around the principles of Islaam. (1) "From the excellence of a persons Islam is his leaving that which does not concern him”

 العناية (attention/ care) and it is: الاهتمام (interest/concern) meaning: That the person abandons what doesn’t concern him in his deen, and his akhirah, and be concerned about the affairs of his deen and the affairs of his akhirah.

 Al-Islaam:هو الاستسلام لله بالتوحيد والانقياد له بالطاعة (Islaam is a complete submission to Allaah with Tawheed and unyielding compliance to Him with obedience) (2)

 “… all that which Allaah legislated therefore is from Islaam, and what he has prohibited from. So avoiding it, is from Islaam. Therefore Islaam: is actions of obedience, and avoiding the disobedience, and prohibitions as in the hadeeth: 

“The Muslim is one who The Muslims are safe from his tongue and hands, and the muhaajir is the one who makes hijrah from what Allaah has prohibited.” (Collected by Bukharee and Muslim from the hadeeth of Abdullah ibn Amr ibn Al-Aasرضي الله عنه) 

Translator: ابو عبد الهادي خليل ديفيس Abu 'AbdilHaadee Khaleel Davis

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(1) There are various statements from the salaf in regards to this principle. From among them are: 

Imam Ahmad said, “The foundations of Islam are three ahadith:
1.The hadith of ‘Umar:‘Verily actions are by intention’ 
2.The hadith of A’ishah: ‘Whoseover introduces into this affair of ours that which is not part it, then it is rejected’ 
3.The hadith of Nu’man ibn Bashir: ‘The halal is clear and the haram is clear’.

 Also: 
 1."Verily actions are by intention."
 2. "From the excellence of a persons Islam, is his leaving that which does not concern him."
 3."The believer will not truly believe, until he likes for his brother that which he likes for himself." 
 4."The halal is clear, and the haram is clear."

 (2) Some scholars add: و براءة من الشرك و أهله (and separation from shirk and it’s people)