Narrated by Abu Huraira (Radhiallaho anho):
Allah’s Messenger (sallallaahu ‘alaihi wasallam) 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.