Sayyidah Asma' bint Abi Bakr (may Allah be pleased with her) said: "In the time of the Messenger of Allah (peace and blessings be upon him) the fast was broken on a cloudy day, then the sun came out."
Hisham was asked: Were they ordered to make it up? He said: "Is there any escape from it? (Meaning it certainly had to be made up.)"
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20717