541 – Ibn ʿUmar (may Allah be pleased with them both) said: I heard the Messenger of Allah ﷺ declare, “When you sight it, begin the fast; and when you sight it, end the fast. If it is hidden from you, then reckon it.” Agreed upon.

Muslim’s wording is: “If it is concealed from you, then reckon it as thirty.”

Al-Bukhārī records: “Then complete the count to thirty.” ⦗1⦘

He also relates in the report of Abū Hurayrah (may Allah be pleased with him): “Complete the count of Shaʿbān to thirty.” ⦗2⦘

— Vocabulary of the ḥadith —
• “When you sight it, begin the fast”: i.e., the crescent; the meaning is: when someone among you sees it in a manner that legally establishes the obligation to fast.

• “Is concealed from you”: with the ghayn quiescent and the mīm kasra, in the passive voice—meaning the crescent is covered and hidden from you. The verb aghmiya is used when something becomes obscure; here it refers to the crescent being veiled by cloud or haze, so complete the count. Some narrations read “fa-in ghumma,” with ḍamma on the ghayn and a doubled mīm, likewise in the passive, meaning it is hidden by cloud and the like.

• “Then reckon it” (fa-qdirū lah): with the dāl either ḍamma or kasra. Al-Miṣbāḥ explains: that is, determine the number of the month and complete Shaʿbān as thirty days.