" لاَ تَصُومُوا حَتَّى تَرَوُا الْهِلاَلَ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ "
It was narrated from Ibn 'Umar that the Messenger of Allah mentioned Ramadan and said: "Do not fast until you see the crescent, and do not stop fasting until you see it, and if it is obscured from you (too cloudy), then work it out."