Narrated `Abdullah bin `Umar: Allah's Apostle 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).
3.131:
Narrated `Abdullah bin `Umar: Allah's Apostle said, "The month
(can be) 29 nights (i.e. days), and do not fast till you see
the moon, and if the sky is overcast, then complete Sha'ban as
thirty days."
3.132:
Narrated Ibn `Umar: The Prophet said, "The month is like this
and this," (at the same time he showed the fingers of both his
hands thrice) and left out one thumb on the third time.
3.133:
Narrated Abu Huraira: The Prophet or Abul−Qasim said, "Start
fasting on seeing the crescent (of Ramadan), and give up
fasting on seeing the crescent (of Shawwal), and if the sky is
overcast (and you cannot see it), complete thirty days of
Sha'ban."
(Sahi Bukhari)