Musnad Ahmad 30:662

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Ibn Abi Anis, from his father, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When the month of Ramadan enters, the gates of mercy are opened, the gates of Hell are closed, and the devils are chained."