The Messenger of Allah — may Allah’s peace and blessings be upon him — mentioned the month of Ramaḍān, and he gestured with his hands, saying: “The month is like this, and this, and this,” then he folded his thumb on the third time, and said: “So fast upon sighting it, and break your fast upon sighting it; and if it is obscured from you, then complete thirty (days).”

Narrated by al-Bukhārī (5302) and Muslim (1080) — and the wording is his — from the ḥadīth of Ibn ʿUmar — may Allah be pleased with them both.

Brief Explanation of the Hadith

Allah created the lunar crescents for great benefits. He made them markers of time for people and for acts of worship — by them are the months known in their beginning and end, and through them are determined the appointed times for fasting, ḥajj, and all other devotional acts.
In this ḥadīth, the Prophet — may Allah’s peace and blessings be upon him — informs that the entry of the lunar month is not established by astronomical calculation, but rather by the legal sighting of the crescent. A lunar month at times consists of thirty days, and at other times twenty-nine. He — may Allah’s peace and blessings be upon him — gestured with his hands three times: the first gesture indicating ten days, the second twenty, and the third thirty. Then, in the third gesture, he folded his thumb — signifying that the month may also consist of twenty-nine days.
He — may Allah’s peace and blessings be upon him — then said: “Fast upon sighting it, and break your fast upon sighting it.” After clarifying to them that the month may be complete or incomplete, he emphasized that both the commencement and conclusion of fasting are contingent upon the legal sighting of the crescent, and that breaking the fast is not permitted until the sighting of the crescent of Shawwāl has been duly confirmed.
He — may Allah’s peace and blessings be upon him — then clarified that if, on the night of the thirtieth of Shaʿbān, something prevents the sighting of the crescent — such as clouds, dust, or anything of that nature — then it becomes obligatory to complete the month of Shaʿbān as thirty days. This is the meaning of his statement: “If it is obscured from you, then complete it as thirty.” The meaning of “taqdīr” here is completion and full estimation, i.e., consider the month complete at thirty days. It is not to be “estimated” by astronomical calculation or conjecture, for the Sharīʿah has tied the ruling to what the eyes can perceive, not to calculations.
Thus, the ḥadīth stands as a foundational principle in the matter of crescent sighting. It establishes that the rulings of the Sharīʿah are built upon manifest signs observable to the general populace, not upon astronomical calculations that most people may be unaware of. In this lies facilitation and mercy for the Ummah.