Hadith

The Holy Prophet (s)

إذَا طَلَعَ هِلاَلُ شَهْرِ رَمَضَانَ فُتِحَتْ أَبْوَابُ اَلسَّمَاءِ وَ أَبْوَابُ اَلْجِنَانِ وَ أَبْوَابُ اَلرَّحْمَةِ وَ غُلِّقَتْ أَبْوَابُ اَلنَّارِ

When the crescent for the month of Ramadan appears,The doors to the heavens, Paradise and mercy are opened. The doors to hellfire are closed

al-Kafi v.4, p.67