(2) بَاب وُجُوبِ صَوْمِ رَمَضَانَ لِرُؤْيَةِ الْهِلَالِ وَالْفِطْرِ لِرُؤْيَةِ الْهِلَالِ وَأَنَّهُ إِذَا غُمَّ فِي أَوَّلِهِ أَوْ آخِرِهِ أُكْمِلَتْ عِدَّةُ الشَّهْرِ ثَلَاثِينَ يَوْمًا
Abdullah b. 'Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying:
The month (of Ramadan) is thus and thus, and thus. i.e. ten, ten and nine.
Reference: Sahih Muslim 1080 k
In-book reference: Book 13, Hadith 14
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2373