Sahih Muslim Book - 13, Hadith - 2514

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى أَخْبَرَنَا إِبْرَاهِيمُ بْنُ سَعْدٍ عَنِ ابْنِ شِهَابٍ عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ عَنْ أَبِى هُرَيْرَةَ - رضى الله عنه - قَالَ قَالَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- « إِذَا رَأَيْتُمُ الْهِلاَلَ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَصُومُوا ثَلاَثِينَ يَوْمًا ».

It was narrated from Abu Hurairah [May Allah be pleased with him] said: “The Messenger of Allah (SAW) said: ‘When you see the crescent then fast, and when you see it, then break the fats, and if it is cloudy than fasts thirty days.’”