كتاب الصيام
Read the English translation first, with the original Arabic directly underneath.
It was narrated from Abu Hurairah that the Messenger of allah sadi:
"When you see the crescent then fast, and when you crescent then fast, and when you see it, stop fasting. If it is obscured from you (too cloudy), then fast thirty days."
إِذَا رَأَيْتُمُ الْهِلاَلَ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَصُومُوا ثَلاَثِينَ يَوْمًا