The Messenger of Allah ﷺ said, "The month is twenty-nine days. Do not begin fasting until you see the crescent moon, and do not break your fast until you see it. If it is obscured from you, then estimate its measure."
أن رسول الله ﷺ قال: الشهر تسع وعشرون، فلا تصوموا حتى تروا، الهلال، ولا تفطروا حتى ترواه، فإن غم عليكم فاقدروا له.
Malik narrated it in the Book of Fasting (2) from Abdullah bin Dinar, from Abdullah bin Umar, as already mentioned. Bukhari narrated it in the Book of Fasting (1907) from Abdullah bin Maslama (that is, Qanabi), Malik narrated to us, with the same wording, except that at the end he said, "If it is obscured from you, then complete the count to thirty." Muslim narrated it in the Book of Fasting (1080/9) through Ismail bin Jafar, from Abdullah bin Dinar, with wording like that of Malik in Muwatta.Ibn Hajar said in Fath al-Bari (4/121), "As for the hadith of Ibn Umar, the narrators from Malik, from Nafi, agreed on his saying, 'So estimate it' ... The narrators from Malik, from Abdullah bin Dinar, also agreed on his saying, 'So estimate it.' Likewise Zafarani and others narrated it from Shafii, meaning from Malik. And likewise Ishaq Harbi and others narrated it in Muwatta from Qanabi. But Rabi bin Sulayman and Muzani narrated it from Shafii, and he said in it, as Bukhari said from Qanabi, 'If it is obscured from you, then complete the count to thirty.'"Bayhaqi said in Marifah, "If the narration of Shafii and Qanabi through these two routes is preserved, then Malik narrated it in both ways." This is the correct interpretation, since it explains his saying ﷺ, "So estimate it," as meaning to complete the count to thirty.Khattabi says in his Maalim, "This is the view of the generality of scholars, and it is confirmed by his ﷺ prohibition of fasting on the day of doubt. Ahmad used to say, if the new moon was not sighted on the twenty-ninth of Shaban because of some obstruction in the sky, people should fast, but if the sky was clear, they should not fast, following the position of Ibn Umar." End of quote.Ibn Abdul-Hadi said in Tanqih (1993), "What the hadiths indicate on this matter, and what the principles require, is that whichever month is obscured should be completed to thirty, whether that is the month of Shaban, the month of Ramadan, or any other. Accordingly, his saying, 'If it is obscured from you, then complete the count,' refers back to both clauses, that is, his saying, 'Fast upon sighting it and break the fast upon sighting it, but if it is obscured from you, then complete the count,' meaning if it is obscured from you regarding your fasting or your breaking of the fast. This is the apparent meaning of the wording, and the rest of the hadiths point to this, such as his saying, 'If it is obscured from you, then estimate it.' The meaning is not 'restrict it,' as some of the companions supposed, but rather it means 'calculate its measure,' for it comes from 'the measure of a thing,' which is the amount of its quantity, and has nothing to do with restriction." End of quote.Ibn Abdul-Hadi also has a fine treatise in which he set out the narrations from Imam Ahmad and established that the correct position among them is that one should not fast on the thirtieth of Shaban, and that the hadiths reported on this subject support this. This treatise is titled Iqamat Burhan ala Adam Wujub Siyam Yawm Thalathin min Shaban.