Sayyiduna Utbah bin Farqad al-Sulami (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) saying: "When the month of Ramadan comes, the gates of Paradise are opened, the gates of Hell are closed, the devils are chained, and a caller calls out: O seeker of good, come forward; and O seeker of evil, desist, until the month comes to an end."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14398