Sahih Al Bukhari – Book of Merits Of The Helpers In Madinah (Ansaar)
Volumn 005, Book 058, Hadith Number 177.
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Narated By 'Umar : The Prophet said, "If anybody has to take an oath, he should swear only by Allah." The people of Quraish used to swear by their fathers, but the Prophet said, "Do not swear by your fathers."