“Indeed, the month is twenty-nine days; therefore, do not fast until you see it, and do not break the fast until you see it. If it is obscured from you, then determine it accordingly.”

Narrated by al-Bukhārī (no. 1900) and Muslim (no. 1080), with the wording being his, from the ḥadīth of Ibn ʿUmar—may Allah be pleased with them both.

Brief Explanation of the Hadith

Allah, the Exalted, has made the crescent moons as markers of time for His servants, by which the commencement and conclusion of the months are known, and upon which great acts of worship—such as fasting, Ḥajj, and others—are dependent.
In this ḥadīth, the Prophet—may Allah’s peace and blessings be upon him—said: “Indeed, the month is twenty-nine,” meaning that the month may be deficient, and one should not assume the necessity of completing it as thirty days every time; rather, it may be twenty-nine. He then made the fasting of Ramaḍān contingent upon sighting its crescent, saying: “So do not fast until you see it,” that is, it is not legislated for you to begin fasting Ramaḍān until its crescent has been confirmed by sighting, as in his statement: “When you see the crescent, then fast.”
From it is derived the prohibition of preceding the fast before the confirmed sighting of the crescent by way of precaution, which is known as the “Day of Doubt.” He then said: “And do not break the fast until you see it,” meaning that it is not legislated for you to exit from Ramaḍān except after the sighting of the crescent of Shawwāl has been established. This indicates that both fasting and breaking the fast are determined by the sighting of the crescent of Ramaḍān and the crescent of Shawwāl. As for astronomical calculation, it is not relied upon in establishing the beginning or end of the month, for the Lawgiver has tied the ruling to the sighting of the crescent, not to calculation. The Prophet, may Allah's peace and blessings be upon him, clarified the ruling when sighting is obstructed, saying: “Then estimate it (the month).” That is, if clouds or dust prevent the crescent from being seen, complete Sha‘bān to thirty days, as other narrations affirm. Hence, “estimate it” indicates completion of thirty days, not imposition of restriction.
The ḥadīth establishes a fundamental principle: that the criterion for the beginning and end of the month is the sighting of the crescent, not astronomical calculations. This is evidenced by the prohibition of fasting on the day of doubt as a precaution, and by the permissibility of naming it “Ramadān” without adding the word “month.”