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), while mentioning fasting, said: "Do not begin fasting until you see the moon, and do not end the fasts until you see the moon. And if it is cloudy, then complete the count by estimation, that is, complete thirty days."

Abu Huraira, may Allah be pleased with him, said: The Messenger of Allah, peace and blessings be upon him, said—or he said: Abu al-Qasim, peace and blessings be upon him, said: "Fast upon sighting the moon and break the fast upon sighting the moon, and if clouds obscure the end of the month, then complete thirty days."

Sayyiduna Ibn Abbas (may Allah be pleased with them both) expressed amazement at a person who fasts for Ramadan in advance (without sighting the moon), and he used to say that the Messenger of Allah (peace and blessings be upon him) said: "Begin fasting when you see the moon and stop fasting when you see the moon, and if clouds obscure you, then complete the count of thirty days."