It is narrated from one of the Companions of the Prophet (peace and blessings be upon him) (may Allah be pleased with him) that he said: We had completed thirty days of Ramadan and began the morning fasting, when two bedouin men came and testified before the Messenger of Allah (peace and blessings be upon him) that they had sighted the new moon the previous evening. So the Messenger of Allah (peace and blessings be upon him) commanded the people, and they broke their fast.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14734