Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates: The Noble Prophet (peace and blessings be upon him) said: "Whoever fasts in Ramadan with faith and seeking reward, his past sins are forgiven." (Imam Ibn Hibban, may Allah have mercy on him, says:): The words "with faith" mean: whoever believes in its obligation, and "seeking reward" means that he is sincere in his action.
Hadith Referenceصحیح ابن حبان / كتاب الصوم / 3432
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط مسلم