Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: In the time of the Messenger of Allah (peace and blessings be upon him), the people differed about (sighting) the new moon. Then a Bedouin came and testified that he had seen it. The Prophet (peace and blessings be upon him) asked: "Do you testify that there is none worthy of worship except Allah and that I am the Messenger of Allah?" He said: Yes. So the Prophet (peace and blessings be upon him) ordered Sayyiduna Bilal (may Allah be pleased with him), and he announced: "Tomorrow is a fast."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11736