‏الصيام : Hadeeth No:2363 Ibn Umar (Allaah be pleased with both of them) reported Allaah's Messenger (sallAllaahu alayhi wa sallam) as saying in connection with Ramadan: Do not fast till you see the new moon, and do not break fast till you see it; but if the weather is cloudy calculate about it....

‏الإيمان : Hadeeth No:213 It is narrated on the authority of Abu Huraira that the Messenger of Allaah (sallAllaahu alayhi wa sallam) observed: Be prompt in doing good deeds (before you are overtaken) by turbulence which would be like a part of the dark night. During (that stormy period) a man would be a Muslim in the morning...