Reported by Tamim ibn Aus ad-Daari (may Allah be pleased with him): The Prophet (ﷺ) said, “The deen (religion) is naseehah (advice, sincerity).” We said, “To whom?” He (ﷺ) said, “To Allah, His Book, His Messenger, and to the leaders of the Muslims and their common folk.” [Muslim]
The Prophet (ﷺ) said: “The deen is naseehah.” This comprehensive word refers to the importance of naseehah in Islam. It is the foundation and the pillar of the Deen. It is also part of Hadith of Jibreel, of which the Prophet (ﷺ) explained about Islam, Iman and Ihsan, where he named them as deen, as he (ﷺ) said: “This is Jibreel, he came to teach you your deen.” This kind of statement is similar to he’s saying (ﷺ): “Al-Hajj is ‘Arafah,” that is because the Wuquf in ‘Arafah is the greatest pillar of Hajj, of which there is no Hajj for the one who misses it.
This comprehensive word refers to the importance of naseehah in Islam. It is the foundation and the pillar of the Deen.
In Mustakhraj Abi ‘Awanah, the Prophet (ﷺ) repeated the sentence “ad-Deenu an-Naseehah” three times, while in Sahih Muslim it is without repetition. When the Companions heard the importance and the significance of naseehah, and it’s great position, they said: “To whom O Messenger of Allah?” So the Prophet (ﷺ) answered with the five categories mentioned in the hadith. It was explained by the people of knowledge about the five. The best explanation is from Abu ‘Amru Ibn Solah in his book Sianatu Sahih Muslim min Ikhlal wal Ghalat, wa Himayatihi minal Isqat was Saqat:
“An-Naseehah is a comprehensive word, indicating the action of the adviser for the one who is advised in a manner which is good intentionally and practically.
An-Naseehah for Allah Tabaraka wa Ta’ala means: to proclaim His Oneness and to attribute Him with the perfect and magnificent attributes, to purify Him from whatever that is contrary and against perfection, to keep away from disobeying Him, to establish obedience and love towards Him with sincerity, to love and hate for His sake, to fight against disbelief and the likes for His sake, and to make dua and urge towards naseehah.
An-Naseehah for His Book (Al-Qur’an) means: to have Iman in it, to venerate and to purify it, to recite it with the proper recitation, to be aware of its commandments and prohibitions, to understand its sciences and parables, to contemplate on its verses, to make dua with it, to protect it from distortions of the extremists and defamations of the disbelievers.
An-Naseehah for His Messenger (ﷺ) is somewhat similar: to have Iman in him and in whatever he comes with, to venerate and dignify him, to adhere in obedience, to revive his Sunnah, to take guidance from his teachings and to propagate it, to take as enemy those whom he takes as enemy, to love whom so ever he loves, to follow his akhlak, to behave according to ethics, to love his family and companions, and so on.
An-Naseehah for the leaders of the Muslims means: to collaborate with them upon the truth and to obey them in it, to exhort and remind them with gentleness and kindness, to stay away from opposing them, to ask Allah to grant them at-Taufiq and diligent in it.
An-Naseehah for the rest of the Muslims (besides the leaders) means: to give guidance to them towards their well-being, to teach them their religion and worldly matters, to conceal their mistakes, to cover their fault, to help and protect them against their enemies, to keep away from deception and envy towards them, to love for them what a person like, to hate for them what a person dislike, and so on.”
The explanation of the great stature of an-Naseehah, and its paramount position in the Deen
The definition of an-Naseehah and whom it is targeted at.
The importance of an-Naseehah towards the five categories mentioned in the hadith.
The persistence of the Companions (may Allah be pleased with them) in knowing the Deen, illustrated from their questioning on for whom is an-Naseehah.
Islam is based on action, as the Messenger (ﷺ) explained an-Naseehah is the Deen.
Excerpts from Fathul Qowiyul Matin (Explanation of An-Nawawi’s 40 Hadith) by As-Sheikh Al-Muhaddith Abdul Muhsin Al-Abbad (may Allah preserve him)