Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Whoever stands (in prayer) on Laylat al-Qadr out of faith and seeking reward, his previous sins will be forgiven; and whoever fasts Ramadan out of faith and seeking reward, his previous sins will be forgiven."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8523