Yahya related to me from Malik from Zayd ibn Aslam from his
brother Khalid ibn Aslam that Umar ibn al-Khattab once broke thefast
on a cloudy day thinking that evening had come and the sun had set.
Then a man came to him and said, "Amir al-muminin, the sun has come
out,'' and Umar said, "That's an easy matter. It was our deduction
(ijtihad)."
Malik said, "According to what we think, and
Allah knows best, what he was referring to when he said, 'That's an
easy matter' was making up the fast, and how slight the effort
involved was and how easy it was. He was saying (in effect), 'We will
fast another day in its place.' "
Muwatta Malik Book 18, Hadith 49