Musnad Ahmad 30:1793

Qutaybah ibn Sa'id narrated to us, 'Abd al-'Aziz ibn Muhammad narrated to us, from Abu Suhayl ibn Malik, from his father, from Abu Hurayrah, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "When Ramadan begins, the gates of the Fire are closed, the gates of Paradise are opened, and the devils are shackled."