On the authority of Abu Hurairah (radiAllahu ‘anhu — may Allah be pleased with him) said the Messenger of Allah (sallallaahu ‘alayhi wa sallam — peace be upon him), “From the perfection of one’s Islam is leaving off that which doesn’t concern him.” [Tirmithi and others]
Shaykh Saalih Aali Shaykh (hafithahullah — may Allah preserve him) said explaining this hadith: The people of knowledge, from them Ibn Abi Zed Al-Qayrawani Al-Maliki, have said, “All good characteristics stem from four ahaadeeth (pl. hadith)”, mentioning this hadith as one of them. This hadith is a foundation from the foundations of character. Imam An-Nowawi (in his book “Imam Nowawi’s 40 Hadith”) chose these hadith to be comprehensive of different subjects; so in every subject there’s a foundational principle.
In his statement (sallallaahu ‘alayhi wa sallam), “From the perfection of one’s Islam is leaving off that which doesn’t concern you”, the word from means that some of what makes one’s Islam perfect is leaving off that which doesn’t concern him. This is apparent in the Arabic language. As for his statement (sallallaahu ‘alayhi wa sallam), “Perfection of one’s Islam”, this sentence is used in numerous ahaadeeth. From them the statement of the Prophet (sallallaahu ‘alayhi wa sallam), “If one of you perfects his Islam, then ever good deed that he does will be multiplied by ten times up to seven hundred times. And ever evil deed that he does he is multiplied like it.” In another narration, “If the servant (of Allah) accepts Islam and perfects it then Allah will forgive all of his evil deeds he’d committed. After that the worth of a good deed will be multiplied ten times to seven hundred times, and evil deeds like that (except what Allah pardons).” So these, and other then them, prove that perfecting one’s Islam is a great level and merit.
Perfecting Islam is something that the scholars have differed upon into different views:
The first view: That the meaning of perfecting Islam is that the servant upholds the obligatory acts and stays away from the prohibited acts. It is the level of the moderate people that come in the statement of Allah:
“Then we caused to inherit the Book those We have chosen of Our servants; and among them is he who wrongs himself, and among them is he who is moderate, and among them is he who is foremost in good deeds by permission of Allah” — Surah Fatir (35) : 32
Therefore, the person who is moderate upholds the obligatory acts, leaves off the prohibited acts, and puts with the obligatory some supererogatory acts; whoever is like that then surely he’s perfected his Islam.
The second view: That the perfection Islam means that the servant be on the level of perfection in ebadah (worship), as it came in the well known hadith of Jibreel wherein: “He said, ‘Tell me about perfection.” He (sallallaahu ‘alayhi wa sallam) said, ‘To worship Allah as if you see Him, and if you don’t see Him then He sees you.’” So, the one who perfects his Islam is the one who’s reached the level of perfection. This second view is apparently complete, but it’s not complete from every aspect.
The third view: That the perfection of Islam is not in one category, rather its in many; so the by the amount [one’s] Islam is perfect he’s have the appropriate merit and reward. This is like his statement (sallallaahu ‘alayhi wa sallam), “If one of you perfects his Islam, then ever good deed that he does will be multiplied by ten times up to seven hundred times. And ever evil deed that he does he is multiplied like it.” They [i.e., the scholars who hold this view] say that the one who accepts Islam and perfects his Islam then he starts at ten multiples of reward. In other words, his good deed is recorded with ten like it, up to seven hundred times. This is according to the level of perfection of Islam, therefore proving the difference in reward according to the difference of Islam. In other words the level of perfection differs and the people who’ve perfected their Islam differ in they’re level of merit and reward applicably. From the reasons it’ll be increased to seven hundred times be by the perfection of Islam being magnificent. This is what Ibn Umar (radiAllahu ‘anhuma — may Allah be pleased with them both) and other then him said about His statement (‘azza wa jal — mighty and majestic):
“Whoever comes [on the Day of Judgment] with a good deed will have ten times the like thereof [to his credit]” — Surah Al-An’aam (6) : 160
He (Ibn Umar) said, “The one who gets his good deed multiplied by ten is for the Bedouin believers. But as for the one who perfected his Islam, then for him is greater as it comes in His statement (subhaanahu wa ta’ala — glory to him the exalted):
“While if there is a good deed, He multiplies it and gives from Himself a great reward.” — Surah An-Nisa (4) : 40
This is a correct judgment because the people in the perfection of their Islam differ.
The people who check, from the people of knowledge, have concluded and clarified that the perfection of Islam has different levels and is not one level. The people of transgression, meaning the people that oppress themselves, are not the same as the people who perfect their Islam.
His statement (sallallaahu ‘alayhi wa sallam), “From the perfection of one’s Islam is leaving off that which does not concern him”, this action — leaving off that which doesn’t concern you — is from perfecting Islam, and this is apparent in all of the levels. So the one who comes with acts of obedience, stays away from the prohibited, busying him with obeying his Lord superseding that which doesn’t concern him. They’re busying themselves with good deeds apparently and hidden superseding worrying about things that don’t concern them.
What is understood from perfection of one’s Islam is placing precedence to what concerns him. And that which doesn’t concern the Muslim from statements that won’t benefit their religion, worldly life, next life, or past, and then leaving it off is from perfecting one’s Islam. This is general and comprehensive to other affairs that don’t benefit. Leaving off that which is does not concern him is a proof that one desires goodness because otherwise you could fall into prohibited actions or leaving off the obligatory.
In conclusion, this hadith is from the great ahaadeeth of character and it is necessary for us and obligatory upon us to be diligent in perfecting out Islam because in it is great merit. From perfection of Islam is leaving off that which does not concern you from speech, listening, not asking that does not have purpose, and not looking into people’s affairs what doesn’t have purpose (this one did, that one didn’t do, this one went, and that one came to the end of it). All of this is shameful and cancels the servant’s perfection of Islam if it overcomes him.
With that we say: In this hadith is a great commandment in this great characteristic of Al-Mustafa (i.e., the Prophet) (sallallaahu ‘alayhi wa sallam). Leaving off that which doesn’t concern you leads to correcting the heart and actions. Some of the Salaf said about people who talk a lot between themselves, “Those people don’t have a lot of actions and have a lot of speech.” People who have a lot of speech without actions, sitting in gatherings for long hours talking don’t benefit from it and this is shameful. If the servant pondered over the obligatory actions will find they’re a lot. Some people over do the permissible things, possibly having with it some prohibited speech and actions, leaving off a lot of obligatory acts. This is not from the characteristics of the student of knowledge. The student of knowledge strives for his actions to always be in what benefits him. In other words, in what he was commanded with in the Shari’ah (legislation) or called to, and leaves off that which doesn’t concern him from speech and actions openly and hidden.
 His full name is Abu Abdullah Muhammad Abdullah ibn Abi Zayd Al-Qayrawani Al-Maliki. He was a major scholar of Maliki Fiqh from Morocco (present day Tunisia) who died 386 H. May Allah have mercy upon him
And I send peace and salutations upon our Prophet Muhammad, his family and his companions.
Summarized from: Sh. Saalih Aali Shaykh’s explanation of Imam Nowawi’s 40 Hadeeth