Narrated `Abdullah bin `Umar:
Allah's Messenger (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).
Reference | : Sahih al-Bukhari 1906 |
In-book reference | : Book 30, Hadith 16 |
USC-MSA web (English) reference | : Vol. 3, Book 31, Hadith 130 |
(deprecated numbering scheme) |