Riyad us Saliheen: Hadith 1224

The Messenger of Allah (ﷺ) said, "Do not observe Saum (fasting) before the advent of Ramadan. Observe Saum at sighting of the crescent of Ramadan and terminate it at sighting the crescent (of Shawwal). If the sky is overcast, complete (the month as) thirty (days)."

[At- Tirmidhi].