Sunan Ibn Majah, Chapter: The Book of Fasting

"My paternal uncles among the Ansar who were among the Companions of the Messenger of Allah (saw) told me: 'The new crescent of Shawwal was covered with clouds, so we fasted the next day. Then some riders came at the end of the day and testified to the Prophet (saw) that they had seen the new crescent the night before. The Messenger of Allah (saw) commanded them to break their fast and to go out to offer the Eid prayer the following morning.'"