It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: When you see the moon, then start fasting, and when you see the moon, then celebrate Eid al-Fitr, and if on any day the sky is cloudy, then complete the count of thirty days.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 14526