Merits of the Helpers in Madinah (Ansaar)

Bukhari :: Book 5 :: Volume 58 :: Hadith 177

Narrated '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. "