A bedouin came to the Prophet (ﷺ) and said: I have sighted the moon. Al-Hasan added in his version: that is, of Ramadan. He asked: Do you testify that there is no god but Allah? He replied: Yes. He 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 said: Bilal, announce to the people that they must fast tomorrow.
Reference: Sunan Abi Dawud 2340 In-book : Book 14, Hadith 28 English translation : Book 13, Hadith 2333
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