Abu Huraira reported God's messenger as saying, "He who fasts during Ramadan with faith and seeking his reward from God will have his past sins forgiven; he who prays during the night in Ramadan with faith and seeking his reward from God will have his past sins forgiven; and he who passes Lailat al-qadr [Night of Decree] in prayer with faith and seeking his reward from God will have his past sins forgiven." (Bukhari and Muslim.)
Hadith Referenceمشكوة المصابيح / كتاب الصوم / 1958
Hadith Gradingالألبانی:مُتَّفَقٌ عَلَيْهِ | زبیر علی زئی:متفق عليه
Hadith Takhrij´تحقيق و تخريج: محدث العصر حافظ زبير على زئي رحمه الله` «متفق عليه، رواه البخاري (37) و مسلم (760/175)»