The Messenger of Allah, peace and blessings be upon him, said:

إِذَا كَانَ أَوَّلُ لَيْلَةٍ مِنْ شَهْرِ رَمَضَانَ صُفِّدَتْ الشَّيَاطِينُ وَمَرَدَةُ الْجِنِّ وَغُلِّقَتْ أَبْوَابُ النَّارِ فَلَمْ يُفْتَحْ مِنْهَا بَابٌ وَفُتِّحَتْ أَبْوَابُ الْجَنَّةِ فَلَمْ يُغْلَقْ مِنْهَا بَابٌ وَيُنَادِي مُنَادٍ يَا بَاغِيَ الْخَيْرِ أَقْبِلْ وَيَا بَاغِيَ الشَّرِّ أَقْصِرْ وَلِلَّهِ عُتَقَاءُ مِنْ النَّارِ وَذَلكَ كُلُّ لَيْلَةٍ

On the first night of the month of Ramadan, the devils are chained and the jinn are restrained, the gates of Hellfire are closed and none of its gates are opened, the gates of Paradise are opened and none of its gates are closed, and a heavenly caller announces: O seeker of good, come near! O seeker of evil, stop short! Allah has those He saves from the Hellfire, and that is during every night.

Source: Sunan al-Tirmidhī 682,