" I heard the messenger of Allah say: 'When you see the crescent then fast, and when you see it, then stop fasting, and if it is obscured from you (too cloudy), then work it out (fro the beginning of the month, to complete thirty days)"' (Using translation from Nasāʾī 2120)
قَالَ رَسُولُ اللهِ ﷺ إِذَا رَأَيْتُمُ الْهِلَالَ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ