From Abu Hurairah (ra) that the Messenger of Allah صلى الله عليه وسلم said:

“When Ramadhaan begins the doors to Jannah are opened and those of Jahannam are locked and the Shayateen are chained up.”

Al Bukhaari (1899), Muslim, An Nisaa’i, Ahmad, Ibn Hibbaan, Ad Daarami with differences in some words.