Abu Hurairah (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) said, "Observe Saum (fast) on sighting the crescent and terminate it on sighting it (the new moon), but if the sky is cloudy before you, then complete the number (thirty days) of the month."[Al-Bukhari and Muslim].
App reference: Book 9, Hadith 231
Arabic/English book reference: Book 9, Hadith 1221