Musnad Ahmad 30:1870

'Affan narrated to us, Hammad bin Zayd narrated to us, Ayyub narrated to us, from Abu Qilabah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said, giving glad tidings to his companions: "There has come to you Ramadan, 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."