3- One of the wisdoms behind this—Allah knows best—is to draw a clear distinction between obligatory worship and voluntary devotion, to meet Ramadan with energy and eagerness, and so that fasting becomes the distinctive emblem of that noble, privileged month.

The Second Hadith

ʿAbd Allah ibn ʿUmar (may Allah be pleased with them both) said: I heard the Messenger of Allah (peace and blessings be upon him) say:

“When you sight it, begin the fast; and when you sight it, break the fast. If it is obscured from you, then calculate for it.”

Terminology:
• “ghumma ʿalaykum” (it is obscured from you): passive construction meaning that a screen—clouds, dust, and the like—has hidden the crescent; the root is gh-m-m, with ghayn ḍammah and a doubled mīm.
• “faqdurū lahu” (then calculate for it): that is, reckon it by computation—complete the number of Shaʿban as thirty days.
Another explanation holds that “aqdirū” means “constrict,” i.e., shorten Shaʿban and treat it as twenty-nine days. The ensuing disagreement is based on these two interpretations.
• Both ḍammah and kasrah are permissible on the dāl of “aqdirū lahu.”
• His words “fast” (faṣūmū) signify that one forms the intention to fast and keeps that intention until the following morning; the same applies to his words “break the fast” (fa-afṭirū).

Overall meaning:
The rulings of the noble sharia rest upon established presumptions, from which one departs only with certainty. Here, the presumption is that Shaʿban continues and that no fast is due so long as it has not completed thirty days—unless the crescent of Ramadan is sighted, proving that the new month has begun. For this reason the Prophet (peace be upon him) tied both the commencement and the conclusion of Ramadan to actual moon-sighting.

If clouds, dust, or anything similar block the view, he instructed the Muslims to calculate its time: complete Shaʿban as thirty days, then fast. This follows the principle, “What already exists remains as it was.”

Scholarly disagreement:
The scholars differed over the ruling of fasting on the thirtieth day of Shaʿban when the crescent is hidden by cloud, haze, or anything else that prevents its sighting.