I am amazed at those who fast before the month begins, when the Messenger of Allah ﷺ said, "When you see the new moon, begin fasting, and when you see it again, break your fast. If it is obscured from you, then complete the count as thirty days."
أخبرنا محمد بن عبد الله بن يزيد قال حدثنا سفيان عن عمرو بن دينار عن محمد بن حنين
عجبت ممن يتقدم الشهر وقد قال رسول الله ﷺ إذا رأيتم الهلال فصوموا وإذا رأيتموه فأفطروا فإن غم عليكم فأكملوا العدة ثلاثين
[Sahih]. Narrated by Ahmad: 1/221, and Humaydi, no. 514, from Sufyan bin Uyayna, from him, and it is in Kubra, no. 2435, and in Musnad: 1/367, and others * Muhammad bin Jubayr means Ibn Mutim, and this is the weaker of the two views, and the hadith has very many corroborating narrations.