The Messenger of Allah ﷺ said, "The month is only twenty-nine days, so do not fast until you see it, and do not break your fast until you see it. If it is obscured from you, then estimate it."Nafi said that whenever twenty-nine days of Shaban had passed, Abdullah would send someone to look for the moon. If it was sighted, that settled the matter. If it was not sighted, and there were no clouds or haze blocking the view, he would begin the next day not fasting. But if clouds or haze obscured the view, he would begin the next day fasting.
حدثنا إسماعيل أخبرنا أيوب عن نافع
قال رسول الله ﷺ إنما الشهر تسع وعشرون فلا تصوموا حتى تروه ولا تفطروا حتى تروه فإن غم عليكم فاقدروا له قال نافع فكان عبد الله إذا مضى من شعبان تسع وعشرون يبعث من ينظر فإن رئي فذاك وإن لم ير ولم يحل دون منظره سحاب ولا قتر أصبح مفطرا وإن حال دون منظره سحاب أو قتر أصبح صائما
Its isnad is sahih. Muslim also narrated it, except for the account of Nafi about the practice of Ibn Umar, which is an addition found in Ahmad, as noted in al-Muntaqa 2104, and see also 3515 and 4300."If it is obscured from you" Ibn Athir said, it is said, "the new moon was obscured from us" when a cloud or something similar blocks its sighting, from the root meaning "I covered the thing" when you conceal it. In the word "obscured," there is an implied pronoun referring to the new moon. It is also possible that "obscured" refers to the circumstance itself, meaning "if you find yourselves in a state of obscurity, then complete the count," with the mention of the new moon omitted because it is understood without being stated."Then estimate for it" Ibn Athir said, meaning estimate for it the number of the month until you complete it as thirty days. It has also been said that it means estimate for it the stations of the moon, for they indicate to you whether the month is twenty-nine or thirty days.Ibn Siraj said, this is an address to those whom Allah has singled out with this knowledge, while His saying "then complete the count" is an address to the general public, who are not skilled in calculating the lunar stations. It is said, "I estimated the matter," using either vowel pattern, when you examine it closely and reflect upon it."Al-Qatr," with two fatha vowels, is the plural of "qatarah," which is the dust-like haze overlaid with a blackness resembling smoke.