Musnad Ahmad 30:663

Ya'qub narrated to us, my father narrated to us, from Salih. Ibn Shihab said: Ibn Abi Unays narrated to me, that his father narrated to him, that he heard Abu Hurayrah saying: The Messenger of Allah, may Allah bless him and grant him peace, said: "When Ramadan enters, the gates of mercy are opened, the gates of Hell are closed, and the devils are chained."