653. Narrated ‘Abdullaah bin ‘Umar (radhi Allaahu anhu) : Allaah’s Messenger (Sallallaahu Alaihi Wasallam) mentioned
Ramadaan and said,
Do not observe Saum (fast) unless you see the crescent (of Ramadaan), and do not give up Saum till you see the crescent (of Shawwaal), and if the sky is overcast (if you cannot see the crescent), then act on estimation (i.e. count Sha’baan as thirty days).
[Saheeh Al-Bukhaari, Hadeeth No. 130, Vol. 3]
Source : The Book Of As-Siyaam (the fasting) – Chapter 2 – It is obligatory to start fasting in Ramadaan with the sighting of new moon and to finish with the sighting of new moon. If the weather is cloudy at the beginning or at the end, then complete thirty days as the period of the month. Al-Lulu Wal-Marjan, Compiled by : Fuwad Abdul-Baqi, Translated by : Dr. Muhammad Muhsin Khan