Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates: I heard the Prophet Muhammad (peace and blessings be upon him) saying regarding Ramadan: "Whoever performs voluntary acts (nawafil) in it with faith and seeking reward, his past sins will be forgiven." (Imam Ibn Hibban, may Allah have mercy on him, says:) The meaning of 'ihtisab' is that the servant, along with obedience, intends to present himself before his Lord, hoping for its acceptance.
Hadith Referenceصحیح ابن حبان / كتاب الصلاة / 2546
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «الإرواء» (906)، «صحيح أبي داود» (1242).فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط مسلم