I heard Allah's Messenger (ﷺ) 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."
Sahih al-Bukhari Book 31, Hadith 1
← Previous Hadith The Prophet (ﷺ) ordered a man from the tribe of Bani Aslam to announce amongst the people that whoever had eaten should fast the rest of the day, and whoever had not eaten should continue his fast, as that day was the day of 'Ashura' .

Sahih al-Bukhari Book 30, Hadith 112 Next Hadith → Allah's Messenger (ﷺ) said, "Whoever prayed at night the whole month of Ramadan out of sincere Faith and hoping for a reward from Allah, then all his previous sins will be forgiven." Ibn Shihab (a sub-narrator) said, "Allah's Messenger (ﷺ) died and the people continued observing that (i.e. Nawafil offered individually, not in congregation), and it remained as it was during the Caliphate of Abu Bakr and in the early days of 'Umar's Caliphate."

Sahih al-Bukhari Book 31, Hadith 2