It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to encourage standing (in prayer) in Ramadan, but did not command it (as an obligation). He (peace and blessings be upon him) said: "Whoever stands (in prayer) in Ramadan with faith and seeking reward, his past sins will be forgiven." The Prophet (peace and blessings be upon him) passed away while the matter remained like that.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4599