Hadith 4:185
Mu'adh ibn Mu'adh narrated to us, from Ash'ath, who said: Al-Hasan used to say regarding the one who broke his fast while he thought that the sun had set, then it became clear to him that it had not set: He said, "It is sufficient for him."
حدثنا معاذ بن معاذ عن اشعث قال كان الحسن يقول فيمن افطر وهو يرى ان الشمس قد غابت ثم استبان له انها لم تغب قال كان يقول اجزأ منه