Sayyiduna Abu Huraira (may Allah be pleased with him) narrates: The Prophet Muhammad (peace and blessings be upon him) said: "Begin fasting upon sighting it (i.e., the new moon), and break your fast (celebrate Eid al-Fitr) upon sighting it. And if the sky is overcast for you, then complete the count of thirty (days), then celebrate Eid al-Fitr."