Abu Wa'il (may Allah have mercy on him) says that Sayyiduna Umar (may Allah be pleased with him) wrote to us while we were at Khaniqin: Indeed, some crescents are bigger than others, so when you see the crescent at the beginning of the day, do not break the fast until two upright witnesses testify that they saw it the day before.
This is an authentic report (athar) from Umar (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8192