It is narrated from Khalid bin Aslam (may Allah have mercy on him) that Umar bin Khattab (may Allah be pleased with him) broke his fast on a cloudy day in Ramadan, thinking that perhaps evening had come and the sun had set. A man came and said, "O Commander of the Faithful! The sun has risen," (meaning it had not yet set). So Umar (may Allah be pleased with him) said: "This matter is easy."
Hadith Referenceمسند الإمام الشافعي / كتاب الصيام / 657
Hadith Takhrijاسناده صحیح اخرجه مالك في الموطاء الصيام، باب ماجاء في قضاء رمضان والكفارات والبيقي 117/4 ۔ وفي المعرفة السنن والآثار له، رقم : (2473)۔