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 ‏"‏ ‏.‏

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

Book Ref: Muwatta Imam Malik Book 18 Hadith 634
Web Ref:  Muwatta Imam Malik Book 18 Hadith 2