Ramadhan : Month of forgiveness

Past Sins
Narrated Abu Huraira(r.a): The Prophet said:"... whoever fasts during Ramadan out of sincere faith and hoping to attain Allah's rewards, then all his past sins will be forgiven." (Sahih Al-Bukhari).

Narrated Abu Huraira: I heard Allah’s Apostle saying regarding Ramadan, “Whoever prayed at night in it (the month of Ramadan) out of sincere Faith and hoping for a reward from Allah, then all his previous sins will be forgiven.”

Abu Hurairah reported that the Prophet (S) said : "The time between the five prayers, two consecutive Friday Prayers, and two cosecutive Ramadans are expiations for all that has happened during that period, provided that one has avoided the grave (major) sins." [Muslim]

Whosoever feeds another who fasted, in order to break the fast at sunset, for the feeder there shall be forgiveness of sins....

Narrated Abu Huraira: The Prophet said, “Whoever established prayers on the night of Qadr out of sincere faith and hoping for a reward from Allah, then all his previous sins will be forgiven; and whoever fasts in the month of Ramadan out of sincere faith, and hoping for a reward from Allah, then all his previous sins will be forgiven.”

Past and Future Sins

Fasting the month of Ramadan with belief and seeking Allahs reward"Whoever fasts Ramadhan out of Iman and seeking Allahs reward then his past and future sins are forgiven." [Hasan, Ahmad]

Performing the night prayer in Ramadan with belief and seeking Allahs reward"Whoever stands (in prayer) in Ramadan out of Iman and seeking Allahs reward then his past and future sins are forgiven." [Saheeh an-Nasaai.]

Performing the night prayer on the Night of Qadar with belief and seeking Allahs reward"It (Laylatul-Qadr) is in Ramadan, and expect it in the last ten days; it is on an odd night: on the 21st, or the 23rd, or the 25th, or the 27th, or the 29th, or on the last night (of Ramadan). He who performs the night prayer on it out of belief and seeking Allah's reward his past and future sins are forgiven." [Ahmad]]