I am amazed at those who fast before the month begins. The Messenger of Allah ﷺ said, "When you see the crescent, fast, and when you see it, break your fast. If it is obscured from you, complete the count as thirty."
عجبت ممن يتقدم الشهر! وقد قال رسول الله ﷺ: إذا رأيتم الهلال فصوموا، وإذا رأيتموه فأفطروا. فإن غم عليكم فأكملوا العدة ثلاثين.
It was narrated by Nasai (2125), Imam Ahmad (3474), and Darimi (1728), all from the hadith of Amr bin Dinar, from Muhammad bin Jubayr, from Ibn Abbas, and he mentioned it, with the wording being that of Nasai. This chain of transmission is sahih.Muhammad bin Jubayr is the son of Mutim bin Adi Nawfali, a trustworthy narrator of known lineage. However, in the printed edition of Nasai, as well as in Musnad Imam Ahmad (1931), Bayhaqi (4/207), and elsewhere, the name appears as "Muhammad bin Hunayn," with a ha and a nun. This person is unknown, and no one narrated from him except Amr bin Dinar. Mizzi, as noted in Tuhfat Ashraf (5/230), and Ibn Hajar in Nukat Ziraf, both concluded that he is in fact Muhammad bin Jubayr, with a jim and a ba.Scholars have discussed this matter at length, each reaching his own conclusion based on the manuscript copies he examined, and Allah knows best.