Abdullah bin Abbas (may Allah be pleased with them both) narrates that a man came to the Messenger of Allah (peace and blessings be upon him) and said: I have seen the moon. The Prophet (peace and blessings be upon him) asked: Do you bear witness that there is no deity except Allah and that Muhammad (peace and blessings be upon him) is His servant and Messenger? He replied: Yes. The Prophet (peace and blessings be upon him) then announced: Observe the fast.