Tameem Ad-Dari reported: The Messenger of Allah, peace and blessings be upon him, said:
إِنَّ الدِّينَ النَّصِيحَةُ
Verily, the religion is sincerity.
We said, “To whom?” The Prophet said:
لِلَّهِ وَكِتَابِهِ وَرَسُولِهِ وَأَئِمَّةِ الْمُؤْمِنِينَ وَعَامَّتِهِمْ وَأَئِمَّةِ الْمُسْلِمِينَ وَعَامَّتِهِمْ
To Allah, His Book, His Messenger, the leaders of the Muslims, and their common people.
Source: Sahih Muslim 55, Grade: Sahih
The word "nasihah" cannot be accurately translated to English because it is a broad concept which cannot be traced in the English language. Some use the term "sincerity" but this is only part of the concept - to negate deception/cheating. According to Imam Ibnu al-Salah, nasihah is "truly seeking the best, in terms of intention and action, for the one whom he is making nasihah to".
This hadith is a profound statement as Sheikh Jamaal al-Din Zarabozo says that in this one brief statement the Prophet, sallallahu 'alayhi wasallam, described the essence of Islam. Nasihah hence forms the bulk and the most important pillar of Islam since it encompasses Islam, Iman and Ihsan.
Nasihah to Allah
The nasihah should be to Allah first. This includes the fulfillment of the obligations in the best way possible (Ihsan).This should be the goal for every Muslim. This also includes striving to get as close to Allah as possible by doing not only the obligations but also the preferable good deeds, by forbidding the forbidden and avoiding the disliked acts.
Nasihah to Allah also involves:
To do nasihah to Allah one should have the correct intention in one's heart to fulfill the rights of Allah, even when one is excused and it is beyond one's ability to perform these obligations. Sometimes a person may not be able to perform an obligation but at least he has good intentions in his heart to fulfill it in the first place.
The actions of the heart (i.e. to have hope in Allah's mercy, to trust Him, to fear Him and to seek refuge in Him), and the actions of the limbs (prayers, Zakah, etc.) also fall under nasihah to Allah.
Honesty is also another aspect of this great concept. In whatever we do we should always be honest with Allah, similarly with ikhlas (sincerity). In fact ikhlas should be the first thing that a Muslim should attain when we talk about nasihah to Allah.
Nasihah to His Book This includes:
Nasihah to His Messenger
Nasihah to the Muslim Leaders
The word 'leader' stands for both ulama and those in authority (at all levels). Nasihah should be given to all leaders, no matter how high or what the ranking is. No one is above the law in Islam and no one is above needing advice. Nasihah is for the benefit of anyone who is in authority. This means that a ruler, leader or scholar should be the first to accept sincere advice.
Making nasihah to Muslim leaders should include:
Nasihah to the Common Folk of the Muslims
From exploring all the obligations mentioned above, we can see that nasihah encompasses everything in Islam, Iman and Ihsan.