وَحَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا شُعْبَةُ، عَنْ مُحَمَّدِ بْنِ زِيَادٍ، قَالَ سَمِعْتُ أَبَا هُرَيْرَةَ، - رضى الله عنه - يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ صُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ غُمِّيَ عَلَيْكُمُ الشَّهْرُ فَعُدُّوا ثَلاَثِينَ ‏"‏ ‏.‏

Abu Huraira reported Allah's Messenger (PBUH) as saying: Observe fast on sighting it (the new moon) and break it on sighting it. But if (due to clouds) the actual position of the month is concealed from you, you should then count thirty (days). Translation Not Available

Book Ref: Sahih Muslim Book 13 Hadith 2380
Web Ref:  Sahih Muslim Book 6 Hadith 1081 c