Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates that a Bedouin came to the Prophet Muhammad (peace and blessings be upon him) and said, "I saw the moon last night." So the Prophet (peace and blessings be upon him) said: "Do you bear witness that there is no true deity except Allah and that Muhammad (peace and blessings be upon him) is His servant and Messenger?" He replied, "Yes." So the Prophet (peace and blessings be upon him) said: "O so-and-so, stand up and announce among the people that they should fast in the morning."

It is narrated through the chain of Zaidah similar to the above, and it is mentioned that the Prophet ( صلی اللہ علیہ وسلم ) instructed Sayyiduna Bilal (may Allah be pleased with him) to announce among the people.