Sayyiduna Abdullah bin Abbas (may Allah be pleased with them) narrates: The Prophet Muhammad (peace and blessings be upon him) said: "Do not start fasting before Ramadan; begin fasting upon sighting it (i.e., the crescent of the first night), and celebrate Eid al-Fitr upon sighting it. And if clouds prevent you (from seeing the crescent), then complete the count to thirty."