كتاب الصيام
18  
Fasting

Yahya related to me from Malik from Abdullah ibn Dinar from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, said, "A month has twenty-nine days in it. Do not start the fast or break it until you see the new moon. If the new moon is obscured from you, then work out (when it should be)."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ الشَّهْرُ تِسْعٌ وَعِشْرُونَ فَلاَ تَصُومُوا حَتَّى تَرَوُا الْهِلاَلَ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ ‏"‏ ‏.‏
USC-MSA web (English) reference : Book 18, Hadith 2
Arabic reference : Book 18, Hadith 634