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 men came and testified that they had seen the crescent the previous evening, so the Messenger of Allah (peace and blessings be upon him) commanded the people "to break their fast."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8189