Abu Dawud Book 7, Hadith Number 2322.

Narated By Mu’awiyah : AbulAzhar al-Mughirah ibn Farwah said: Mu’awiyah stood among the people at Dayr Mustahill lying at the gate of Hims. He said: O people, we sighted the moon on such-and-such day. We shall fast in advance. Anyone who likes to do so may do it. Malik ibn Hubayrah as-Saba’i stood up and asked: Mu’awiyah, did you hear the Apostle of Allah (PBUH) say something (about this matter), or is this something on the basis of your opinion? He replied: I heard the Apostle of Allah (PBUH) as saying: Fast the month (in the beginning) and in the last.