A bedouin came to the Prophet صلى الله عليه وسلم and said: “I have sighted the moon.” Al-Hasan added in his version: “that is, of Ramadan.” He صلى الله عليه وسلم asked: “Do you testify that there is no god but Allah?” He replied: “Yes.” He صلى الله عليه وسلم again asked: “Do you testify that Muhammad is the Messenger of Allah?” He replied: “Yes,” and he testified that he had sighted the moon. He صلى الله عليه وسلم said: “Bilal, announce to the people that they must fast tomorrow.”

Abu Dawud HADITH_No 2333. Narrated by Abdullah ibn Abbas