The Messenger of Allah ﷺ mentioned Ramadan and said, "Do not begin fasting until you see the crescent moon, and do not break the fast until you see it. If it is hidden from you by clouds, then estimate its appearance."
أن رسول الله ﷺ ذكر رمضان، فقال: لا تصوموا حتى تروا الهلال، ولا تفطروا حتى تروه، فإن غم عليكم فاقدروا له.
Malik narrated it in "Fasting" (1) from Nafi, from Abdullah bin Umar, and he mentioned it. Bukhari narrated it in "Fasting" (1906), and Muslim in "Fasting" (1080), both of them through the route of Malik, with the same chain.