Ali bin Hanzalah (may Allah have mercy on him) narrates from his father, and his father was a friend of Sayyiduna Umar (may Allah be pleased with him). He says: Once in Ramadan I was with Sayyiduna Umar (may Allah be pleased with him); he broke his fast and the people broke their fast too. Then the muezzin went up to call the adhan and said: "O people, here is the sun; it has not yet set." Sayyiduna Umar (may Allah be pleased with him) said: "May Allah suffice us against your evil; we did not send you to watch for this." Then Sayyiduna Umar (may Allah be pleased with him) said: "Whoever of you has broken his fast, let him fast a day in its place."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8014