Narrated `Abdullah bin `Umar: Allah’s Messenger (peace be upon him) 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.”
(Sahih al-Bukhari, Vol. 3, Book 30, Hadith 16)
