I heard Allah's Messenger (ﷺ) saying, "When you see the crescent (of the month of Ramadan), start fasting, and when you see the crescent (of the month of Shawwal), stop fasting; and if the sky is overcast (and you can't see it) then regard the month of Ramadan as of 30 days."
Sahih al-Bukhari Book 30, Hadith 10
← Previous Hadith Allah's Messenger (ﷺ) said, "When the month of Ramadan starts, the gates of the heaven are opened and the gates of Hell are closed and the devils are chained."

Sahih al-Bukhari Book 30, Hadith 9 Next Hadith → The Prophet (ﷺ) said, "Whoever established prayers on the night of Qadr out of sincere faith and hoping for a reward from Allah, then all his previous sins will be forgiven; and whoever fasts in the month of Ramadan out of sincere faith, and hoping for a reward from Allah, then all his previous sins will be forgiven."

Sahih al-Bukhari Book 30, Hadith 11