‏الصيام : Hadeeth No:2380 Abu Huraira reported Allaah's Messenger (sallAllaahu alayhi wa sallam) as saying: Observe fast on sighting it (the new moon) and break it on sighting it. But if (due to clouds) the actual position of the month is concealed from you, you should then count thirty (days)....

‏البر والصلة والآداب‏ : Hadeeth No:6249 Ibu 'Umar reported Allaah's Messenger (sallAllaahu alayhi wa sallam) as saying: Oppression is the darkness on the Day of Resurrection....