The Prophet ﷺ said, "Do not begin fasting until you see it, and do not break your fast until you see it. If it is obscured from you, then estimate for it."
أخبرنا عمرو بن علي قال حدثنا يحيى قال حدثنا عبيد الله قال حدثني نافع
عن النبي ﷺ قال لا تصوموا حتى تروه ولا تفطروا حتى تروه فإن غم عليكم فاقدروا له
Its isnad is sahih.Ahmad narrated it, 2/13, from Yahya Qattan, with the same chain, and Muslim, in the Book of Fasting, chapter on the obligation of fasting Ramadan upon sighting the crescent, etc., hadith no. 1080, from the hadith of Ubaydullah bin Umar with the same chain, with other wordings conveying the same meaning, and Bukhari, hadith no. 1906 (see the previous hadith), through another route from Nafi with the same chain.