The Messenger of Allah ﷺ mentioned Ramadan and said, "Do not fast until you see the crescent, and do not break your fast until you see it. If it is obscured from you, then estimate it."
أخبرنا محمد بن سلمة والحارث بن مسكين قراءة عليه وأنا أسمع واللفظ له عن ابن القاسم عن مالك عن نافع
أن رسول الله ﷺ ذكر رمضان فقال لا تصوموا حتى تروا الهلال ولا تفطروا حتى تروه فإن غم عليكم فاقدروا له
Bukhari narrated it in the Book of Fasting, chapter on the saying of the Prophet ﷺ, "When you see the new moon, then fast, and when you see it, then break your fast," hadith no. 1906, and Muslim in the Book of Fasting, chapter on the obligation of fasting Ramadan upon sighting the new moon, etc., hadith no. 3/1080, from the hadith of Malik through this chain. It is also found in the Muwatta (Yahya's recension), 1/86, and in the Kubra, hadith no. 2431.