“When you see the new crescent then fast, and when you see it then stop fasting. If it is cloudy then fast thirty days.” (Using translation from Ibn Mājah 1655)  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ «إِذَا رَأَيْتُمُ الْهِلَالَ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَصُومُوا ثَلَاثِينَ يَوْمًا»  

When you see the new moon, observe fast, and when you see it (again) then break it, and if the sky is cloudy for you, then calculate it. (Using translation from Muslim 1080g)  

سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ «إِذَا رَأَيْتُمُوهُ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ»  

Messenger of Allah ﷺ mentioned Ramadan and said, "Do not fast unless you see the crescent (of Ramadan), and do not give up fasting till you see the crescent (of Shawwal), but if the sky is overcast (if you cannot see it), then act on estimation (i.e. count Sha'ban as 30 days). (Using translation from Bukhārī 1906)  

أَنَّ رَسُولَ اللهِ ﷺ ذَكَرَ رَمَضَانَ فَقَالَ «لَا تَصُومُوا حَتَّى تَرَوُا الْهِلَالَ وَلَا تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ»