´It was narrated that Abu Hurairah said:` "The Messenger of Allah said: The month may be twenty-nine days or it may be thirty. When you see it, then stop fasting, and it is obscured from you (too cloudy), then complete the count."'
Hadith ReferenceSunan an-Nasa'i / The Book of Fasting / Hadith: 2140
Hadith Gradingالألبانی:صحيح الإسناد | زبیر علی زئی:صحيح