Good Manners and Form (Al-Adab)

Bukhari :: Book 8 :: Volume 73 :: Hadith 129

Narrated Ibn 'Umar:

that he found 'Umar bin Al-Khattab in a group of people and he was swearing by his father. So Allah's Apostle called them, saying, "Verily! Allah forbids you to swear by your fathers. If one has to take an oath, he should swear by Allah or otherwise keep quiet."