It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Holy Prophet (peace and blessings be upon him) said: "Begin fasting upon sighting the moon, and celebrate Eid upon sighting the moon. If the moon is not visible and the sky is cloudy, then complete the count of thirty days."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 9472
Hadith Gradingحکم دارالسلام:حدیث صحیح ، خ : 1909 ، م : 1081 ، وھذا إسناد ضعیف ، الحجاج بن أرطاۃ مدلس ، وقد عنعن