The Book of Fasting
13/2. Chapter:
The obligation to fast Ramadan when the crescent is sighted, and to break the fast when the crescent is sighted, and that if it is cloudy at the beginning or end of the month, then the month should be completed as thirty days.
Sahih Muslim
وَحَدَّثَنِي حَجَّاجُ بْنُ الشَّاعِرِ، حَدَّثَنَا حَسَنٌ الأَشْيَبُ، حَدَّثَنَا شَيْبَانُ، عَنْ يَحْيَى، قَالَ وَأَخْبَرَنِي أَبُو سَلَمَةَ، أَنَّهُ سَمِعَ ابْنَ عُمَرَ، - رضى الله عنهما - يَقُولُ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " الشَّهْرُ تِسْعٌ وَعِشْرُونَ " .
Ibn Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying: The month may consist of twenty-nine days.
Reference : Sahih Muslim 1080jIn-book reference : Book 13, Hadith 13USC-MSA web (English) reference : Book 6, Hadith 2372