Musnad Ahmad 30:2370

Isma'il narrated to us, from Ayyub, from Abu Qilabah, from Abu Hurayrah, who said: When Ramadan arrived, the Messenger of Allah (peace and blessings of Allah be upon him) said: "Ramadan has come to you, a blessed month. Allah has obligated its fasting upon you. In it, the gates of Paradise are opened, the gates of Hellfire are closed, and the devils are chained. In it is a night that is better than a thousand months. Whoever is deprived of its good has indeed been deprived."