Allah Almighty says: {The believers are but brothers} [Surat al-Hujurāt: 10] Allah Almighty also says as He reports the speech of Nūh (Noah) (peace be upon him): {and I give you sincere advice} [Surat al-A‘rāf: 62] and the speech of Hūd (Heber) (peace be upon him): {and I am your sincere adviser} [Surat al-A‘rāf: 68]
Nasīhah is to extend good to others, which entails that the giver of Nasīhah likes for his brother to be in a good state and invite him to what is good and makes it clear to him. The opposite of Nasīhah is cunning, cheating, betrayal, and deception.
1) Nasīhah is a fruit of religion-inspired brotherliness; rather, it is among the essential requirements of such brotherliness.
2) One should be a giver of Nasīhah to his brothers, showing to them what is good so that the faith-based bond of brotherliness is realized.
As for the relevant Hadīths:
181/1- First Hadīth: Abu Ruqayyah, Tamīm ibn Aws al-Dāri (may Allah be pleased with him) reported that the Prophet (may Allah’s peace and blessings be upon him) said: “The religion (Islam) is Nasīhah (sincerity).” We (the Companions) said: “To whom?” He said: “To Allah, to His book, to His Messenger, to the imams (scholars and rulers) of Muslims, and to their commoners.” [Narrated by Muslim]
1) Nasīhah to Allah Almighty means to be sincere to Him, achieve servitude to Him out of love and glorification, to have protective zeal for the sacred ordinances of Allah Almighty, defend His religion and call people to believe in it.
2) Nasīhah to the Book of Allah means realization of its words, by recitation and contemplation, spreading its correct meanings among the Muslims, and adhering to its teachings by doing what is obligatory, avoiding what is prohibited and believing its reports.
3) Nasīhah to His Messenger (may Allah’s peace and blessings be upon him) means complete belief in his message, deeming truthful all what he informed about, true compliance with his guidance, defending and protecting his Sharia, combating religious innovations and those who follow them, respecting, honoring, loving and defending the Companions (may Allah be pleased with them).
4) Nasīhah to Muslim imams means to respect the scholars, be keen to learn the knowledge they possess, and avoid pursuing their slips and mistakes because they are not infallible. Rather, one should defend them. As for the Muslim rulers, Nasīhah to them means to overlook their downsides and offer to them advice as much as he can.
5) Nasīhah to common Muslims means that one should like for them what he likes for himself, guide them to good, direct them to the truth and be merciful to them.
182/2- Second Hadīth: Jarīr ibn ‘Abdullah (may Allah be pleased with him) reported: “I gave the Messenger of Allah (may Allah’s peace and blessings be upon him) my pledge to establish the prayer, pay Zakah, and extend Nasīhah to every Muslim.” [Narrated by Al-Bukhāri and Muslim]
183/3- Third Hadīth: Anas (may Allah be pleased with him) reported that the Prophet (may Allah’s peace and blessings be upon him) said: “None of you is a believer till he loves for his (Muslim) brother what he loves for himself.” [Narrated by Al-Bukhāri and Muslim]
1) The Hadīths encourage extending Nasīhah to all Muslims, relative or non-relative, young or old, male or female.
2) Wishing for fellow Muslims what one wishes for himself is an aspect of Nasīhah to Muslims.
3) A person’s faith is not complete until he likes for his fellow Muslims what he likes for himself.
The negation of faith in the aforementioned Hadīth: “None of you is a believer...” means negating complete faith, not the origin of it. In fact, faith has an origin and branches, and it increases and decreases. A guided believer is the one who seeks to maintain and increase his faith. {Do you not see how Allah compares a good word to a good tree? Its root is firm and its branches reach the sky, yielding its fruit all the times, by the permission of its Lord} [Surat Ibrāhīm: 24-25]