The Messenger of Allah ﷺ said, "Do not anticipate the month by fasting a day or two before it, unless it falls on a day one of you regularly fasts. Do not fast until you see the new moon, and then fast until you see it again. If clouds obscure it, complete the count as thirty days, then break the fast. The month may be twenty-nine days."
قال رسول الله ﷺ: لا تقدموا الشهر بصيام يوم ولا يومين، إلا أن يكون شيء يصومه أحدكم، ولا تصوموا حتى تروه، ثم صوموا حتى تروه، فإن حال دونه غمامة، فأتموا العدة ثلاثين، ثم أفطروا. والشهر تسع وعشرون.
Narrated by Abu Dawud (2327), Tirmidhi (688), and Nasai (2129, 2130), all of them through various routes from Simak bin Harb, from Ikrima, from Ibn Abbas, and he mentioned it, the wording here being that of Abu Dawud. The wording of the other two is similar, except that they did not mention his statement, "then break your fast." Tirmidhi said, "hasan sahih." Abu Dawud said, "Hatim bin Abu Saghira, Shuba, and Hasan bin Salih narrated it from Simak with the same meaning."I say, in his statement there is an indication of what has been said regarding Simak's narration from Ikrima, namely that it contains inconsistency, except for what Shuba and Sufyan narrated from him, for their reports from him are sahih and sound, as Yaqub bin Shayba said.Shuba's hadith was narrated by Ibn Khuzayma in his Sahih (1912), and from him Ibn Hibban (3590), and Hakim (1/425), from Simak, who said, "I visited Ikrima on the day when there was doubt about Ramadan, and he was eating. He said, 'Come closer and eat.' I said, 'I am fasting.' He said, 'By Allah, you will come closer.' I said, 'Then tell me a hadith.' He said, 'Ibn Abbas told us that the Messenger of Allah ﷺ said, "Do not go out to look for the new moon of the month, but fast when you sight it and break your fast when you sight it. If clouds or dust conceal it from you, then complete the count to thirty.""'"Hakim said, "sahih in isnad, though the two of them (Bukhari and Muslim) did not record it with this wording."