Sahih Al Bukhari – Book of Good Manners And Form (Al-Adab)

Volumn 008, Book 073, Hadith Number 129.
—————————————–
Narated By 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."