Bukhari :: Book 3 :: Volume 31 :: Hadith 214
Narrated Ziyad bin Jubair:
A man went to Ibn 'Umar I. and said, "A man vowed to fast one day (the sub-narrator thinks that he said that the day was Monday), and that day happened to be 'Id day." Ibn 'Umar said, "Allah orders vows to be fulfilled and the Prophet forbade the fasting on this day (i.e. Id)."
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