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, then fast until you see it again. If clouds obscure it, complete the count as thirty days, then break your fast. The month may be twenty-nine days."
حدثنا الحسن بن علي حدثنا حسين عن زائدة عن سماك عن عكرمة
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
Its chain is daif. Tirmidhi narrated it in the Book of Fasting, chapter on what has been said that fasting begins with the sighting of the crescent and breaking the fast likewise, no. 688, and Nasai, no. 2131, from the hadith of Simak bin Harb with the same wording. Tirmidhi said, hasan sahih. I say, its chain is daif, since the chain of Simak from Ikrima is a daif chain.