"D not fast until you see it , and do not stop fasting until you see it, and if it is obscured from you (too cloudy), then work it out."  

عَنِ النَّبِيِّ ﷺ قَالَ لاَ تَصُومُوا حَتَّى تَرَوْهُ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ  

"The Messenger of Allah mentioned the crescent and said: 'When you see it, fast and when you see it, stop fasting, and if it is obscured from you (too cloudy), then count thirty days."'  

ذَكَرَ رَسُولُ اللَّهِ ﷺ الْهِلاَلَ فَقَالَ إِذَا رَأَيْتُمُوهُ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَعُدُّوا ثَلاَثِينَ