The Messenger of Allah ﷺ said, "Do not fast before Ramadan. Fast when you sight the moon and break your fast when you sight it. If clouds obscure it, complete thirty days."
أخبرنا قتيبة قال حدثنا أبو الأحوص عن سماك عن عكرمة
قال رسول الله ﷺ لا تصوموا قبل رمضان صوموا للرؤية وأفطروا للرؤية فإن حالت دونه غياية فأكملوا ثلاثين
[Sahih] Reported by Tirmidhi, Fasting, chapter on fasting being determined by sighting the new moon and breaking the fast accordingly, h: 688, from Qutayba, with this wording, and he said, "hasan sahih." It is also in Kubra, h: 2440, though its chain is daif, and the hadith has sahih corroborating narrations.