(5) باب هَلْ يُقَالُ رَمَضَانُ أَوْ شَهْرُ رَمَضَانَ وَمَنْ رَأَى كُلَّهُ وَاسِعًا
Narrated Ibn `Umar:
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."
Reference: Sahih al-Bukhari 1900
In-book reference: Book 30, Hadith 10
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 31, Hadith 124