Musnad Ahmad 30:1563

Sulayman narrated to us, he said: Isma'il informed us, Abu Suhayl Nafi' ibn Malik ibn Abi 'Amir informed me, from his father, from Abu Hurayrah, that the Prophet, peace and blessings of Allah be upon him, said: "When Ramadan comes, the gates of Paradise are opened, the gates of the Fire are closed, and the devils are chained."