It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Fast upon sighting the moon and celebrate Eid al-Fitr upon sighting the moon. If clouds come between you and the moon, then complete the count of thirty and do not start the new month."
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 1985
Hadith Gradingحکم دارالسلام:صحيح، سماك عن عكرمة مضطربة ، لكن سماكا توبع.