Zayd bin Aslam narrates from his brother Khalid bin Aslam that Umar bin al-Khattab (may Allah be pleased with him) broke his fast in Ramadan on a cloudy day, thinking that evening had come and the sun had set. Then a man came to him and said: "O Commander of the Believers, the sun has appeared." Umar (may Allah be pleased with him) said: "The matter is a small one, and we did exercise our judgement." Imam al-Shafi'i (may Allah have mercy on him) says: He is to make up one day in its place.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8012