Sayyiduna Tamim Dari (may Allah be pleased with him) narrates that the Noble Prophet (peace and blessings of Allah be upon him) said: "Be aware! Religion is sincerity. Be aware! Indeed, religion is sincerity. Be aware! Indeed, religion is sincerity." The people asked: "O Messenger of Allah (peace and blessings of Allah be upon him)! For whom?" The Noble Prophet (peace and blessings of Allah be upon him) replied: "For Allah, His Book, His Messenger, the leaders of the Muslims, and their common people (to be sincere to them)."
Hadith Referenceصحیح ابن حبان / كتاب السير / 4575
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «الإرواء» (26)، «غاية المرام» (332): م.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح