Ibn 'Abbas (May Allah be pleased with them) reported:
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].
App reference: Book 9, Hadith 235
Arabic/English book reference: Book 9, Hadith 1225