Abu Bakr bin Abi Shayba أبو بكر بن أبي شيبة · The Book of Fasting

'When Ramadan begins, the gates of Paradise are opened, the gates of Hell are closed, and the devils are chained up."

: "The Messenger of Allah said: 'Fast when you see it and stop fasting when you see it, and if it is obscured from you (too cloudy), then count it as thirty (days).'"