Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said when the month of Ramadan arrived:

قَدْ جَاءَكُمْ رَمَضَانُ شَهْرٌ مُبَارَكٌ افْتَرَضَ اللَّهُ عَلَيْكُمْ صِيَامَهُ تُفْتَحُ فِيهِ أَبْوَابُ الْجَنَّةِ وَيُغْلَقُ فِيهِ أَبْوَابُ الْجَحِيمِ وَتُغَلُّ فِيهِ الشَّيَاطِينُ فِيهِ لَيْلَةٌ خَيْرٌ مِنْ أَلْفِ شَهْرٍ مَنْ حُرِمَ خَيْرَهَا قَدْ حُرِمَ

The month of Ramadan has come, a blessed month in which Allah Almighty has obligated you to fast. In it the gates of the heavens are opened, the gates of Hellfire are closed, the devils are chained, and in it is a night that is better than a thousand months. Thus, whoever is deprived of its good is truly deprived.

Source: Musnad Aḥmad 7148, Grade: Sahih