CHAPTER: Whoever Said That If It Is Obscured From You (The Crescent), Then Fast Thirty Days.
Narrated Abdullah ibn Abbas: The Prophet صلی اللہ علیہ وسلم said: Do not fast one day or two days just before Ramadan except in the case of a man who has been in the habit or observing a fast (on that day); and do not fast until you sight it (the moon). Then fast until you sight it. If a cloud appears on that day (i. e. 29th of Ramadan) then complete the number thirty (days) and then end the fasting: a month consists of twenty-nine days.