Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.109.

Abu Hurairah reported that the Prophet, upon whom be peace, said: “The blessed month has come to you. Allah has made fasting during it obligatory upon you. During it, the gates to Paradise are opened and the gates to hellfire are locked, and the devils are chained. There is a night [during this month] which is better than a thousand months. Whoever is deprived of its good is really deprived [of something great].” This is related by Ahmad, an-Nasa’i, and al-Baihaqi.

‘Arfajah testifies to this: “We were with ‘Utbah ibn Farqad while he was discussing Ramadan. A companion of the Prophet entered upon the scene. When ‘Utbah saw him, he became shy and stopped talking. The man [the companion] spoke about Ramadan, saying: ‘I heard the Messenger of Allah say during Ramadan: “The gates of Hell are closed, the gates of Paradise are opened, and the devils are in chains. An angel calls out: ‘O you who intend to do good deeds, have glad tidings. O you who intend to do evil, refrain, until Ramadan is completed.'”

Muslim relates that Abu Hurairah reported the Prophet saying: “The time between the five prayers, two consecutive Friday prayers, and two consecutive Ramadans are expiations for all that has happened during that period, provided that one has avoided the grave sins.”

Abu Sa’id al-Khudri reported that the Prophet, upon whom be peace, said: “Whoever fasts the month of Ramadan, obeying all of its limitations and guarding himself against what is forbidden, has in fact atoned for any sins he committed before it.” Ahmad and alBaihaqi related this hadith with a good chain.

Abu Hurairah reported that the Prophet, upon whom be peace, said: “Whoever fasts the month of Ramadan with faith and seeks Allah’s pleasure and reward will have his previous sins forgiven.” This hadith is related by Ahmad and the compilers of the sunan.