Tameem Ad-Daari (radiyallaahu-anhu) reported that the Messenger of Allaah (sallal-laahu-alayhi-wasallam) said: ”Indeed, the religion is Naseehah, the religion is Naseehah, the religion is Naseehah; they said: To who O Messenger of Allaah? He said: ‘’To Allaah, His Messenger, to the leaders of the Muslims and their common people.”[Reported by Muslim]
Al-Allaamah Saaleh Al-Fawzaan (may Allaah preserve him) said:
The meaning of ‘Naseehah’ is Al-Khuloos (purity). It is said that something is ‘Naasih’ meaning: It is free from deceit. And it is said: ‘A-sa-lun Naasihun’ (Pure Honey), ‘La-ba-nun Naasihun (Pure Milk), meaning: Free from deceit and bad mixtures. This is how it is (with regards) to the religion of Islaam, for indeed it is free from every falsehood and from every deception, cheating and treachery. It is a pure religion (and) an unadulterated religion.
Likewise, the Muslim’s apparent (affairs) are the same as his hidden (affairs), upon purity and safe from evil manners, treachery, betrayal and other than that. As for the one who cheats, deceives and plots, or his apparent (affairs) are in opposition to his hidden (affairs), this trait is not from the Religion. The Prophet (sallal-laahu-alayhi-wasallam) confined the religion within (Naseehah –purity-sincerity), and the confinement of something necessitates that something else cannot enter into it. [For Further details see: Minhatur-Rabbaaniyyah Fee Sharhi Ar-ba’een An-Nawawiyyah: page: 112].