Hadith 506330
Chapter 18: Fasting - كتاب الصيام
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم ذَكَرَ رَمَضَانَ فَقَالَ " لاَ تَصُومُوا حَتَّى تَرَوُا الْهِلاَلَ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ " .
Yahya related to me from Malik from Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, once mentioned Ramadan and said, "Do not begin the fast until you see the new moon, and do not break the fast (at the end of Ramadan) until you see it. If the new moon is obscured from you, then work out (when it should be)."
Reference Muwatta Malik 506330
USC-MSA web (English) reference : Book 18, Hadith 1