Tamim Dari (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) said: "Religion is the name of sincerity." Religion is the name of being sincere: to Allah, to His Prophet (peace and blessings be upon him), to the leaders of the Muslims, and to the common Muslims."
Hadith Referenceمسند الإمام الشافعي / كتاب فضائل قريش وغيرهم وأبواب متفرقة / 1805
Hadith Takhrijاخرجه مسلم الايمان، باب بيان أن الدين النصيحة (55)۔