It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Do not fast until you see the moon, and do not celebrate Eid until you see it. If clouds cover you, then estimate it."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5294
Hadith Gradingحکم دارالسلام:إسناده صحيح ، خ : 1906 ، م : 1080 .