"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." (Using translation from Nasāʾī 2122)  

عَنِ النَّبِيِّ ﷺ قَالَ «لَا تَصُومُوا حَتَّى تَرَوْهُ وَلَا تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ»  

Observe fast when you see it (the new moon) and break fast when you see it (the new moon of Shawwal), but when (the actual position of the month is) concealed from you (on account of cloudy sky), then count thirty days. (Using translation from Muslim 1081d)  

ذَكَرَ رَسُولُ اللهِ ﷺ الْهِلَالَ فَقَالَ «إِذَا رَأَيْتُمُوهُ فَصُومُوا فَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَعُدُّوا ثَلَاثِينَ»