Observing the moon

عَنْ عَبْدِ اللّٰه بْنِ عُمَرَ رَضِیَ اللّٰه عَنْہ أنَّ رَسُولَ اللّٰهِ صَلَّى اللّٰهُ عَلَيْهِ وَسَلَّم ذَكَرَ رَمَضانَ فَقالَ

لاَ تَصُومُوا حَتّى تَرَوُا الهِلالَ، ولاَ تُفْطِرُوا حَتّى تَرَوْهُ، فَإنْ غُمَّ عَلَيْكُمْ فاقْدُرُوا لَهُ

Hazrat Abdullah bin Umar^ra narrated, “Allah’s Messenger^sa 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, then work out its [sighting].’”

(Sahih al-Bukhari, Kitab as-sawmi, Bab qawlin n-nabiyyi^sa ‘idha ra’aytumu l-hilala fa sumu wa idha ra’aytumuhu fa ’aftiru)