Religion is sincere advice. الدِّينُ النَّصِيحَةُ.

The essence of sincere advice is to reconcile people, derived from the word for sewing, which means to mend what is broken. This is based on a hadith narrated from the Messenger of God, peace and blessings be upon him, which reads: "They said: 'To whom, O Messenger of God?' He said: 'To God, His Messenger, the leaders of the Muslims, and their general public.'" Scholars have said: Sincere advice to God means that the servant dedicates his actions to God alone. Sincere advice to His Messenger means that his heart is pure in accepting the call of prophethood and does not harbor any opposition to it. Sincere advice to the Muslims means that they do not distinguish themselves from him in any way. It has also been said that sincere advice to the leaders of the Muslims means that their unity is not broken, nor their rulings disobeyed.
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The hadith “religion is advice” does not mean merely giving advice, but rather that the essence of religion is based on sincerity, honesty, and good will towards God, His Messenger, and Muslims. Therefore, there is no English proverb that matches it exactly, but the closest equivalents according to meaning are:
Honesty is the best policy.
Honesty is the best policy.
- It is close to the meaning of sincerity and good intentions, although it is narrower than the meaning of the hadith.
True faith is sincere devotion.
True faith is sincerity and loyalty.
- An interpretive translation that is closer to what is intended.
Religion is sincere counsel.
— This is the common literal translation of the hadith.
The essence of religion is sincerity.
The essence of religion is sincerity.
- This is closest to the meaning of the hadith according to scholars, because advice here means sincerity and good will.
If you are looking for an English equivalent in meaning rather than in pronunciation, the best form is:
The essence of religion is sincerity.
If you want to translate the hadith itself, the usual formula is:
Religion is sincere counsel.
Or:
Religion is sincerity.

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الأصل في النصيحة التلفيق بين الناس، من النصح وهو الخياطة، وذلك أن تلفق بين التفاريق، وهذا من حديثٍ يروى عن رسول الله صلى الله عليه وسلم، وتمامه "قالوا: لمن يا رسول الله؟ قال: لله ولرسوله ولأئمة المسلمين وعامتهم" قالت العلماء: النصيحة لله أن يُخْلِصَ العبدُ العملَ لله، والنصيحة لرسوله أن يَصْفُوَ قلبُه في قبول دعوة النبوّة ولا يضمر خلافها، والنصيحة للمسلمين أن لا يتميزوا عنه في حال من الأحوال، وقيل: النصيحة لأءمة المسلمين أن لا تَشُقَّ عَصَاهم، ولا يعقَّ فتواهم.