Sahih al-Bukhari #1903
Sahih (Authentic)
Fasting
إِذَا دَخَلَ شَهْرُ رَمَضَانَ فُتِّحَتْ أَبْوَابُ السَّمَاءِ وَغُلِّقَتْ أَبْوَابُ جَهَنَّمَ وَسُلْسِلَتِ الشَّيَاطِينُ
Allah's Messenger (ﷺ) said, 'When the month of Ramadan starts, the gates of heaven are opened and the gates of hell are closed and the devils are chained.'
Chain of Narration (Isnad)
Narrated Abu Huraira