The Messenger of Allah ﷺ said, "When Ramadan arrives, the gates of mercy are opened, the gates of Hell are closed, and the devils are chained up."
حدثنا إبراهيم بن إسحاق حدثنا ابن المبارك عن يونس عن الزهري قال أخبرني ابن أبي أنس أن أباه حدثه أنه
قال رسول الله ﷺ إذا كان رمضان فتحت أبواب الرحمة وغلقت أبواب جهنم وسلسلت الشياطين
Its isnad is sahih. Bukhari related it, 3/32, in "Fasting," "Is it said Ramadan," and Muslim, 2/758, no. 1079, in "Fasting," "The merit of the month of Ramadan," and Tirmidhi, 3/57, no. 682, and Nasai, 4/126, no. 2097, and Ibn Majah, 1642, 1643.