The Messenger of Allah ﷺ said, "When the month of Ramadan begins, the gates of heaven are opened, the gates of Hell are closed, and the devils are chained up."
قال رسول الله ﷺ: إذا دخل شهر رمضان فتحت أبواب السماء، وغلقت أبواب جهنم، وسلسلت الشياطين.
Bukhari narrated it in the Book of Fasting (1899), and Muslim in the Book of Fasting (1079), both through Ibn Shihab Zuhri, who said, "Ibn Abu Anas, the freedman of the Taymis, told me that his father told him that he heard Abu Hurairah say" (then he mentioned it). Ibn Abu Anas is Nafi bin Malik bin Abu Amir.