I heard the Messenger of Allah ﷺ say, "When you see the crescent moon, begin fasting, and when you see it again, break your fast. If it is obscured from you, then estimate its position."
أخبرنا الربيع بن سليمان قال حدثنا ابن وهب قال أخبرني يونس عن ابن شهاب قال حدثني سالم بن عبد الله أن
سمعت رسول الله ﷺ يقول إذا رأيتم الهلال فصوموا وإذا رأيتموه فأفطروا فإن غم عليكم فاقدروا له
Muslim narrated it, 8/1080, see the previous hadith, from the report of Abdullah bin Wahb. Bukhari narrated it in the Book of Fasting, chapter on whether one should say "Ramadan" or "the month of Ramadan," and on those who considered all such usage permissible, hadith 1900, from the report of Ibn Shihab Zuhri, with the same wording. It is also found in Al-Kubra, hadith 2430.