مُسْنَدُ أَحْمَدَ
Musnad Ahmad 1984
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that "Ikrimah said: I heard Ibn "Abbas say: The Messenger of Allah (ﷺ) said: "Fast when you see it and break the fast when you see it. And if it is too cloudy, complete the number [of days in the month] as thirty. And do not anticipate the month." Hatim said: i.e., the number of days in Sha"ban.
Musnad Ahmad 1985Sahih]