A companion narrates that the Messenger of Allah, peace and blessings be upon him, said: Do not begin fasting before the month starts; rather, fast when the count of the previous month is completed or the new moon is sighted. Then continue fasting until the count of Ramadan is completed or the new moon is sighted.
Hadith Referenceالفتح الربانی / كتاب الصيام / 3681
Benefits: … The meaning of this hadith is that if the crescent moon is sighted on the 29th, then that is correct; otherwise, one should wait for the completion of 30 days.