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."
Hadith ReferenceSahih Bukhari / The Book of As-Saum (The Fasting). / Hadith: 1907
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة