Abdullah bin Abbas (may Allah be pleased with them) narrates that a Bedouin came to the Messenger of Allah (peace be upon him) and said: I have seen the moon. The Prophet (peace be upon him) said: Do you bear witness that there is no god but Allah and that I am the Messenger of Allah? He said: Yes. The Prophet (peace be upon him) said: O Bilal! Announce among the people that they should fast tomorrow.