On the authority of Tameem Daree (May Allah be pleased with him):
The Prophet (May Allah bless him and grant him peace) said, “The deen (religion) is naseehah (advice, sincerity).” We said, “To whom?” He (May Allah bless him and grant him peace) said, “To Allah, His Book, His Messenger, and to the leaders of the Muslims and their common folk.”
The word "naseehah" (advice, sincerity) means “impartial”, “pure” or “loyal” in the dictionary. In the tradition, it is that one person gives good guidance to the other, with sincere intentions and loyalty. The Prophet (May Allah bless him and grant him peace) said that the deen is naseehah which refers that the main strength of religion is exhortation. It's like saying, "Hajj is the arafah." That is, the strength of the Hajj is to stay in Mount Arafah, the main pillar.
The Companions said, "For whom should we give advice?" The Messenger of Allah (May Allah bless him and grant him peace) explained that there are five things in vigour of the religion of Allah.
1. “For the sake of Allah”, that is, to exhort to believe in Allah, to give thanks to Him and to execute His commandments.
2. “For His Book” - that is, the Book of Allah - to exhort to study the Quran, to adhere to it, and to encourage people to do the same.
3. “For His Messenger”, that is, to exhort to follow the Prophet (May Allah bless him and grant him peace), to adhere to his Shariah and Sunnah and to call others to it.
4. “For the imams of Muslims”. It is through counseling for them, obeying, and inviting people to do the same.
5. “For the Muslims public.” It is to advise all Muslims the happiness of the two worlds, that is, the way of Islam.
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