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»