It is narrated from a Companion (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Do not begin the next month until the counting is complete or you have sighted the moon, then observe the fast. In the same way, do not celebrate Eid al-Fitr until the counting is complete or you have sighted the moon.
Hadith Referenceمسند احمد / تتمہ مُسْنَدِ الْكُوفِيِّينَ / 18825