Bulugh al-Maram #760

Ibn 'Umar (RAA) narrated, ‘I heard the Messenger of Allah (ﷺ) say, "Start fasting after you have seen it (i.e. the new crescent) and end the fast (at the end of the month) when you see it. If it is overcast and cannot be seen, then estimate its sighting (i.e. presume that Sha‘ban lasted for thirty days)." Agreed upon. Muslim narrated, "If it is too overcast to be seen then presume that it (Sha'ban) lasted for thirty days."In another narration by al-Bukhari, "wait, until thirty days of it (Sha’ban) have passed."