Abu Wa'il (may Allah have mercy on him) says: The crescent of Ramadan appeared to us while we were at Khaniqin; some of us fasted and some of us broke the fast. Then a letter from Umar (may Allah be pleased with him) came to us: When you see the crescent during the day, do not break the fast until two Muslim witnesses testify that they saw it the day before. The Shaykh says: By this he means the crescent at the end of Ramadan.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8191