مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2327
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn ‘Abbas (رضي الله عنه) said: The Messenger of Allah (ﷺ) said: "Fast when you see it [the new moon] and break the fast when you see it, and if clouds prevent you from seeing it, then complete the number [of days, in the month]; the month is twenty-nine days,” meaning that it may be shorter [than thirty days].
Musnad Ahmad 2335Saheeh]