Rib'i bin Hirash narrates from one of the Companions (may Allah be pleased with him) that the people differed over the last day of Ramadan. Then two Bedouins came and testified before the Prophet (peace and blessings be upon him) that they had seen the crescent the day before, so he (peace and blessings be upon him) "commanded the people to break their fast and to come to the Eid prayer-ground the next day."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8199