Abu Dawud Book 7, Hadith Number 2333.

Narated By Abdullah ibn Abbas : A bedouin came to the Prophet (PBUH) 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 Apostle 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.