Abu Wa'il (may Allah have mercy on him) says: The letter of Sayyiduna Umar (may Allah be pleased with him) came to us at Khaniqin: the new moon is bigger in some cases than in others, so when you see the new moon at the beginning of the day, do not break your fast until two witnesses testify that they saw it the previous day.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7984