كتاب الصوم
Read the English translation first, with the original Arabic directly underneath.
Narrated 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 Messenger of Allah (ﷺ) say something (about this matter), or is this something on the basis of your opinion? He replied: I heard the Messenger of Allah (ﷺ) as saying: Fast the month (in the beginning) and in the last.
صُومُوا الشَّهْرَ وَسِرَّهُ