Sayyiduna Abu Wa'il (may Allah be pleased with him) says: The letter of Sayyiduna Umar (may Allah be pleased with him) came to us while we were at Khaniqin: "The new moons, some of them are bigger than others; so when you see the new moon in the daytime, do not break your fast until evening comes, unless two Muslim men testify that they sighted it the previous evening."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7982